diff --git a/sound.cpp b/sound.cpp
index 87840c2..d803d98 100644
--- a/sound.cpp
+++ b/sound.cpp
@@ -3,17 +3,19 @@
 #include "timed.h"
 #include "sound.h"
 
+static Sound *s;
+
 #if defined(DAC_SOUND) && defined(ESP_PLATFORM)
 #include <driver/dac.h>
 
 static portMUX_TYPE mux = portMUX_INITIALIZER_UNLOCKED;
-
 static dac_channel_t channel;
 
-static volatile const uint8_t *_bytes;
-static volatile unsigned _size, _off;
-
 void IRAM_ATTR timer_callback() {
+	s->on_tick();
+}
+
+void IRAM_ATTR Sound::on_tick() {
 	portENTER_CRITICAL_ISR(&mux);
 
 	if (_off < _size)
@@ -32,6 +34,7 @@ void Sound::begin(unsigned pin, unsigned freq) {
 	else if (pin == 26)
 		channel = DAC_CHANNEL_2;
 
+	s = this;
 	timer_create(freq, &timer_callback);
 }
 
diff --git a/sound.h b/sound.h
index a3a6ed5..b4af0cd 100644
--- a/sound.h
+++ b/sound.h
@@ -5,6 +5,11 @@ class Sound {
 public:
 	void begin(unsigned pin, unsigned freq);
 	const uint8_t *play(const uint8_t *bytes, unsigned size);
+	void on_tick();
+
+private:
+	volatile const uint8_t *_bytes;
+	volatile unsigned _size, _off;
 };
 
 #endif