diff --git a/bios.cpp b/bios.cpp
index 89a7683..d3dbdab 100644
--- a/bios.cpp
+++ b/bios.cpp
@@ -8,7 +8,6 @@
 #include "cpu.h"
 
 #ifdef TEENSYDUINO
-#include <TeensyThreads.h>
 #include <Bounce2.h>
 #include "teensy-paddles.h"
 extern Bounce resetButtonDebouncer;
@@ -303,7 +302,7 @@ uint8_t BIOS::GetAction(int8_t selection)
 #endif
 	   ) {
 #ifdef TEENSYDUINO
-      threads.delay(10);
+      delay(10);
 #else
       usleep(100);
 #endif
@@ -315,7 +314,7 @@ uint8_t BIOS::GetAction(int8_t selection)
       // wait until it's no longer pressed
       while (resetButtonDebouncer.read() == HIGH)
 	;
-      threads.delay(100); // wait long enough for it to debounce
+      delay(100); // wait long enough for it to debounce
       // then return an exit code
       return ACT_EXIT;
     }
diff --git a/globals.h b/globals.h
index d340f73..e995cc2 100644
--- a/globals.h
+++ b/globals.h
@@ -55,9 +55,4 @@ extern bool g_invertPaddleY;
 
 extern char debugBuf[255];
 
-#ifdef TEENSYDUINO
-#include <TeensyThreads.h>
-extern Threads::Mutex spi_lock;
-#endif
-
 #endif
diff --git a/teensy/fscompat.h b/teensy/fscompat.h
index 9b944f5..0fc0830 100644
--- a/teensy/fscompat.h
+++ b/teensy/fscompat.h
@@ -9,7 +9,6 @@
 
 #include "globals.h"
 #include <Arduino.h>
-#include <TeensyThreads.h>
 
 #define open(path, flags, perms) g_filemanager->openFile(path)
 #define close(filedes) g_filemanager->closeFile(filedes)
@@ -17,9 +16,8 @@
 #define read(filedes,buf,nbyte) g_filemanager->read(filedes,buf,nbyte)
 #define lseek(filedes,offset,whence) g_filemanager->lseek(filedes,offset,whence)
 
-Threads::Mutex serlock;
 static char fsbuf[200];
-#define printf(x, ...) {sprintf(fsbuf, x, ##__VA_ARGS__); serlock.lock(); Serial.println(fsbuf); Serial.flush(); Serial.send_now(); serlock.unlock();}
-#define fprintf(f, x, ...) {sprintf(fsbuf, x, ##__VA_ARGS__); serlock.lock(); Serial.println(fsbuf); Serial.flush(); Serial.send_now(); serlock.lock();}
-#define perror(x) {serlock.lock();Serial.println(x);Serial.flush(); Serial.send_now(); serlock.unlock();}
+#define printf(x, ...) {sprintf(fsbuf, x, ##__VA_ARGS__); Serial.println(fsbuf); Serial.flush(); Serial.send_now();}
+#define fprintf(f, x, ...) {sprintf(fsbuf, x, ##__VA_ARGS__); Serial.println(fsbuf); Serial.flush(); Serial.send_now();}
+#define perror(x) {Serial.println(x);Serial.flush(); Serial.send_now();}
   
diff --git a/teensy/teensy-filemanager.cpp b/teensy/teensy-filemanager.cpp
index 8871bed..405c599 100644
--- a/teensy/teensy-filemanager.cpp
+++ b/teensy/teensy-filemanager.cpp
@@ -2,11 +2,8 @@
 #include <wchar.h>
 #include "teensy-filemanager.h"
 #include <string.h> // strcpy
-#include <TeensyThreads.h>
 #include "teensy-println.h"
 
-Threads::Mutex fslock;
-
 TeensyFileManager::TeensyFileManager()
 {
   numCached = 0;
@@ -23,8 +20,6 @@ TeensyFileManager::~TeensyFileManager()
 
 int8_t TeensyFileManager::openFile(const char *name)
 {
-  Threads::Scope locker(fslock);
-  
   if (cacheFd != -1) {
     cacheFile.close();
     cacheFd = -1;
@@ -56,7 +51,6 @@ int8_t TeensyFileManager::openFile(const char *name)
 
 void TeensyFileManager::closeFile(int8_t fd)
 {
-  Threads::Scope locker(fslock);
   if (cacheFd != -1) {
     cacheFile.close();
     cacheFd = -1;
@@ -92,8 +86,6 @@ void TeensyFileManager::closeDir()
 // suffix may be comma-separated
 int16_t TeensyFileManager::readDir(const char *where, const char *suffix, char *outputFN, int16_t startIdx, uint16_t maxlen)
 {
-  Threads::Scope locker(fslock);
-
   // First entry is always "../" if we're in a subdir of the root
   if (startIdx == 0 || !outerDir) {
     if (outerDir)
@@ -220,7 +212,6 @@ void TeensyFileManager::seekToEnd(int8_t fd)
 
 int TeensyFileManager::write(int8_t fd, const void *buf, int nbyte)
 {
-  Threads::Scope locker(fslock);
   // open, seek, write, close.
   if (fd < 0 || fd >= numCached) {
     return -1;
@@ -249,7 +240,6 @@ int TeensyFileManager::write(int8_t fd, const void *buf, int nbyte)
 
 int TeensyFileManager::read(int8_t fd, void *buf, int nbyte)
 {
-  Threads::Scope locker(fslock);
   // open, seek, read, close.
   if (fd < 0 || fd >= numCached) {
     return -1;
@@ -276,7 +266,6 @@ int TeensyFileManager::read(int8_t fd, void *buf, int nbyte)
 
 int TeensyFileManager::lseek(int8_t fd, int offset, int whence)
 {
-  Threads::Scope locker(fslock);
   if (whence == SEEK_CUR && offset == 0) {
     return fileSeekPositions[fd];
   }
diff --git a/teensy/teensy-println.cpp b/teensy/teensy-println.cpp
index a073ce4..8c0b368 100644
--- a/teensy/teensy-println.cpp
+++ b/teensy/teensy-println.cpp
@@ -3,12 +3,12 @@
 namespace arduino_preprocessor_is_buggy {
   
   bool serialavailable() {
-    Threads::Scope locker(getSerialLock());
+    //    Threads::Scope locker(getSerialLock());
     return Serial.available();
   }
   
   char serialgetch() {
-    Threads::Scope locker(getSerialLock());
+    //    Threads::Scope locker(getSerialLock());
     return Serial.read();
   }
 
diff --git a/teensy/teensy-println.h b/teensy/teensy-println.h
index 3e8cdf8..3ccca27 100644
--- a/teensy/teensy-println.h
+++ b/teensy/teensy-println.h
@@ -1,6 +1,5 @@
 #include <Arduino.h>
 #include <utility>
-#include <TeensyThreads.h>
 
 // cf. https://forum.pjrc.com/threads/41504-Teensy-3-x-multithreading-library-first-release/page6
 
@@ -12,10 +11,12 @@ namespace arduino_preprocessor_is_buggy {
     // used for suppressing compiler warnings
     template<class T> void silence(T&&) {};
 
+  /*
     inline Threads::Mutex& getSerialLock() {
         static Threads::Mutex serial_lock;
         return serial_lock;
     }
+  */
 
   bool serialavailable();
   char serialgetch();
@@ -29,12 +30,12 @@ namespace arduino_preprocessor_is_buggy {
     }
 
     template<class... args_t> void print(args_t... params) {
-        Threads::Scope locker(getSerialLock());
+      //        Threads::Scope locker(getSerialLock());
         silence(expand{ (print_fwd(params), 42)... });
     }
   
     template<class... args_t> void println(args_t... params) {
-        Threads::Scope locker(getSerialLock());
+      //        Threads::Scope locker(getSerialLock());
         silence(expand{ (print_fwd(params), 42)... });
         Serial.println();
 	Serial.flush();
diff --git a/teensy/teensy-speaker.cpp b/teensy/teensy-speaker.cpp
index 8380697..c66a4ec 100644
--- a/teensy/teensy-speaker.cpp
+++ b/teensy/teensy-speaker.cpp
@@ -1,28 +1,49 @@
 #include <Arduino.h>
-#include <TeensyThreads.h>
 #include "teensy-speaker.h"
 #include "teensy-println.h"
-#include <i2c_device.h>
+#include <Audio.h>
+#include <SPI.h>
+
+TeensyAudio audioDriver;
+//AudioMixer4             mixer2;                 //xy=280,253
+AudioMixer4             mixer1;                 //xy=280,175
+AudioOutputI2S          i2s;                    //xy=452,189
+
+AudioConnection         patchCord1(audioDriver, 0, mixer1, 0);
+//AudioConnection         patchCord2(audioDriver, 0, mixer2, 0);
+//AudioConnection         patchCord3(mixer2, 0, i2s, 1);
+AudioConnection         patchCord4(mixer1, 0, i2s, 0);
+//const float     t_ampx  = 0.8;
+//const int       t_lox   = 10;
+//const int       t_hix   = 22000;
+//const float     t_timex = 10;           // Length of time for the sweep in seconds
 
 #include "globals.h"
 
-I2CDevice dac = I2CDevice(Master, 0x60, _BIG_ENDIAN);
+//#define BUFSIZE 4096
+//EXTMEM uint32_t toggleBuffer[BUFSIZE]; // cycle counts at which state toggles
+//uint16_t headptr, tailptr;
 
-Threads::Mutex togmutex;
-#define BUFSIZE 4096
-EXTMEM uint32_t toggleBuffer[BUFSIZE]; // cycle counts at which state toggles
-uint16_t headptr, tailptr;
-uint32_t lastCycleCount, toggleAtCycle;
+// Ring buffer that we fill with 44.1kHz data
+#define RINGBUFSIZE 4096
+EXTMEM short sampleRingBuffer[RINGBUFSIZE];
+volatile uint16_t sampleHeadPtr = 0;
+volatile uint16_t sampleTailPtr = 0;
+volatile uint32_t lastFilledTime = 0;
+
+volatile uint32_t lastSampleNum = 0;
+
+bool toggleState = false;
 
 // How many cycles do we run the audio behind? Needs to be more than our bulk
 // cycle count.
-#define CYCLEDELAY 100
+//#define CYCLEDELAY 100
 
 TeensySpeaker::TeensySpeaker(uint8_t sda, uint8_t scl) : PhysicalSpeaker()
 {
   toggleState = false;
   mixerValue = numMixed = 0;
-  Master.begin(1000000); // 100000 or 400000 or 1000000
+  AudioMemory(8);
 }
 
 TeensySpeaker::~TeensySpeaker()
@@ -31,18 +52,40 @@ TeensySpeaker::~TeensySpeaker()
 
 void TeensySpeaker::begin()
 {
-  lastCycleCount = g_cpu->cycles;
+  mixer1.gain(0, 0.5f); // left channel
+  
+  lastFilledTime = g_cpu->cycles;
+  sampleHeadPtr = sampleTailPtr = 0;
   toggleState = false;
-  memset(toggleBuffer, 0, sizeof(toggleBuffer));
-  headptr = tailptr = 0;
+  //  memset(toggleBuffer, 0, sizeof(toggleBuffer));
+  //  headptr = tailptr = 0;
+  lastSampleNum = 0;
 }
 
 void TeensySpeaker::toggle(uint32_t c)
 {
-  Threads::Scope lock(togmutex);
-  // Queue the speaker toggle time; maintainSpeaker will pick it up
-  toggleBuffer[tailptr++] = c; tailptr %= BUFSIZE;
- 
+  // Figure out when the last time was that we put data in the audio buffer;
+  // then figure out how many audio buffer cycles we have to fill from that
+  // CPU time to this one.
+#if 1
+  __disable_irq();
+
+  // We expect to have filled to this cycle number...
+  uint32_t expectedCycleNumber = (float)c  * (float)AUDIO_SAMPLE_RATE_EXACT / (float)g_speed;
+
+  // and we have filled to cycle number lastFilledTime. So how many do we need?
+  uint32_t audioBufferSamples = expectedCycleNumber - lastFilledTime;
+
+  if (audioBufferSamples > RINGBUFSIZE)
+    audioBufferSamples = RINGBUFSIZE;
+  for (int i=0; i<audioBufferSamples; i++) {
+    sampleRingBuffer[sampleTailPtr++] = toggleState ? (32767/2) : (-32767/2); // FIXME: appropriate value?
+    sampleTailPtr %= RINGBUFSIZE;
+  }
+  toggleState = !toggleState;
+  lastFilledTime = expectedCycleNumber;
+  __enable_irq();
+#endif
 }
 
 void TeensySpeaker::maintainSpeaker(uint32_t c, uint64_t microseconds)
@@ -52,25 +95,13 @@ void TeensySpeaker::maintainSpeaker(uint32_t c, uint64_t microseconds)
 
 void TeensySpeaker::maintainSpeaker()
 {
-  // This is called @ SAMPLERATE (8k, as of this writing) and looks for
-  // any transitions that have passed before sending data to the DAC.
-  // The idea is that this will be called fast enough for the given number
-  // of cycles that we run behind, so that the buffer can't overflow.
-  //
-  // at 8kHz, that means that .000125 seconds have passed since our last
-  // call; where the CPU has executed about 128 instructions in the same
-  // time. Therefore, it can't have toggled more than 128 times. We're
-  // also trying to stay 100 cycles "behind", so it's possible that we have
-  // 228 cycles difference between an event that just fired-and-queued,
-  // and where we need to catch up to in this loop.
-
-  // First, reconcile the "correct" toggleState. We pretend it's currently
-  // some time in the past, based on our cycle delay. In theory (as above),
-  // this shouldn't be more than about 228 cycles off (maybe slightly more,
-  // since we actually run cycles in batches).
+  // This is called @ 44100Hz, which is the sample rate for the
+  // Teensy4 (#define AUDIO_SAMPLE_RATE_EXACT 44100.0f). We fill a FIFO 
+  // that is then drained by update(). In theory, as long as we don't fall
+  // 128 cycles behind, it should be okay, I think (b/c AUDIO_BLOCK_SAMPLES
+  // is 128 on the Teensy 4).
+#if 0
   uint32_t curTime = g_cpu->cycles - CYCLEDELAY;
-  // And then find any events that should have happened, accounting for them:
-  togmutex.lock();
   while (headptr != tailptr) {
     if (curTime >= toggleBuffer[headptr]) {
       toggleState = !toggleState;
@@ -80,11 +111,7 @@ void TeensySpeaker::maintainSpeaker()
       break;
     }
   }
-  togmutex.unlock();
-
-  // Now we can safely update the DAC based on the current toggleState
-  uint16_t v = (toggleState ? 0xFFF : 0x000);
-  //  dac.write((uint8_t) ((v >> 8) & 0xFF), (uint8_t) (v & 0xFF), true);
+#endif
 }
 
 void TeensySpeaker::beginMixing()
@@ -97,3 +124,54 @@ void TeensySpeaker::mixOutput(uint8_t v)
   // unused
 }
 
+void TeensyAudio::update(void)
+{
+  audio_block_t *block;
+  short *bp;
+
+  // Grab a block and we'll fill it up. It needs AUDIO_BLOCK_SAMPLES short values
+  // (which is 128 on the Teensy 4).
+  block = allocate();
+  if (block) {
+    bp = block->data;
+#if 1
+    uint32_t underflow = 0;
+    for (int i=0; i<AUDIO_BLOCK_SAMPLES; i++) {
+      static short lastValue = 0;
+
+      if (sampleHeadPtr == sampleTailPtr) {
+	//	bp[i] = lastValue; // underflow: just repeat whatever old data we have
+	// FIXME: trend toward zero, maybe?
+	bp[i] = lastValue;
+	underflow++;
+      } else {
+	lastValue = sampleRingBuffer[sampleHeadPtr++];
+	bp[i] = lastValue;
+	sampleHeadPtr %= RINGBUFSIZE;
+      }
+
+    }
+#else
+    // Fill in the AUDIO_BLOCK_SAMPLES samples of data, pull them from the FIFO
+    memset(bp, 0, AUDIO_BLOCK_SAMPLES * sizeof(short));
+#endif
+    if (underflow) {
+      println("U ", underflow);
+    }
+    transmit(block, 0);
+    release(block);
+  }
+
+#if 0
+  if (sampleHeadPtr == sampleTailPtr) {
+    // The FIFO is empty, so reset...
+    if (g_cpu) {
+      lastFilledTime = g_cpu->cycles;
+    } else {
+      lastFilledTime = 0;
+    }
+    // FIXME:
+    //    lastSampleNum = 0;
+  }
+#endif
+}
diff --git a/teensy/teensy-speaker.h b/teensy/teensy-speaker.h
index ef0ad86..a7a98e7 100644
--- a/teensy/teensy-speaker.h
+++ b/teensy/teensy-speaker.h
@@ -1,11 +1,19 @@
 #ifndef __TEENSY_SPEAKER_H
 #define __TEENSY_SPEAKER_H
 
+#include <AudioStream.h>
 #include "physicalspeaker.h"
 #include <MCP492X.h>
 
 #define SAMPLERATE 44100
 
+class TeensyAudio : public AudioStream {
+ public:
+  TeensyAudio(void) : AudioStream(0, NULL) {}
+
+  virtual void update(void);
+};
+
 class TeensySpeaker : public PhysicalSpeaker {
  public:
   TeensySpeaker(uint8_t sda, uint8_t scl);
@@ -21,7 +29,7 @@ class TeensySpeaker : public PhysicalSpeaker {
   virtual void mixOutput(uint8_t v);
 
  private:
-  bool toggleState;
+  //  bool toggleState;
 
   uint32_t mixerValue;
   uint8_t numMixed;
diff --git a/teensy/teensy.ino b/teensy/teensy.ino
index b477090..9490792 100644
--- a/teensy/teensy.ino
+++ b/teensy/teensy.ino
@@ -1,7 +1,7 @@
 #include <Arduino.h>
+#include <Audio.h>
 #include <SPI.h>
 #include <TimeLib.h>
-#include <TeensyThreads.h>
 #include <Bounce2.h>
 #include "bios.h"
 #include "cpu.h"
@@ -16,7 +16,7 @@
 #include "teensy-prefs.h"
 #include "teensy-println.h"
 
-#define DEBUG_TIMING
+//#define DEBUG_TIMING
 
 #define THREADED if (1)
 
@@ -69,7 +69,7 @@ void onKeyrelease(int unicode)
 void setup()
 {
   Serial.begin(230400);
-#if 0
+#if 1
   // Wait for USB serial connection before booting while debugging
   while (!Serial) {
     yield();
@@ -165,9 +165,9 @@ void setup()
   println("free-running");
   Serial.flush();
 
-  threads.setMicroTimer(); // use a 100uS timer instead of a 1mS timer
+//  threads.setMicroTimer(); // use a 100uS timer instead of a 1mS timer
   //  threads.setSliceMicros(5);
-  threads.addThread(runDebouncer);
+//  threads.addThread(runDebouncer);
 }
 
 // FIXME: move these memory-related functions elsewhere...
@@ -228,43 +228,13 @@ void biosInterrupt()
   g_keyboard->maintainKeyboard();
 }
 
-void runSpeaker(uint32_t now)
-{
-  static uint32_t spk_nextResetMicros = 0;
-  static uint32_t spk_refreshCount = 0;
-  static uint32_t spk_microsAtStart = micros();
-  static uint32_t spk_microsForNext = 0;
-
-  THREADED {
-    if (now >= spk_microsForNext) {
-      spk_refreshCount++;
-      spk_microsForNext = spk_microsAtStart + ((float)1000000.0*((float)spk_refreshCount/(float)SAMPLERATE));
-      ((TeensySpeaker *)g_speaker)->maintainSpeaker();
-    }
-    
-    if (now >= spk_nextResetMicros) {
-#ifdef DEBUG_TIMING
-      float pct = (100.0 * (float)spk_refreshCount) / (float)SAMPLERATE;
-      sprintf(debugBuf, "Speaker running at %f%% [%lu]", pct, spk_refreshCount);
-      println(debugBuf);
-#endif      
-
-      spk_microsAtStart = now;
-      spk_refreshCount = 0;
-      spk_nextResetMicros = spk_microsAtStart + 1000000;
-      spk_microsForNext = spk_microsAtStart + ((float)1000000.0*((float)spk_refreshCount/(float)SAMPLERATE));
-    }
-    
-  }
-}
-
 void runMaintenance(uint32_t now)
 {
   static uint32_t nextRuntime = 0;
   
   THREADED {
     if (now >= nextRuntime) {
-      nextRuntime = now + 100000000; // FIXME: what's a good time here
+      nextRuntime = now + 100000; // FIXME: what's a good time here? 1/10 sec?
 
       if (!resetButtonDebouncer.read()) {
 	// This is the BIOS interrupt. We immediately act on it.
@@ -334,7 +304,8 @@ void runDebouncer()
       nextRuntime = millis() + 10;
       resetButtonDebouncer.update();
     } else {
-      threads.yield();
+    yield();
+//      threads.yield();
     }
   }
 }
@@ -373,7 +344,6 @@ void loop()
   uint32_t now = micros();
   runCPU(now);
   runDisplay(now);
-  runSpeaker(now);
   runMaintenance(now);
 }