Use the volatile keyword with all inline assembly to make sure the optimizer doesn't remove it.

apple2048/Makefile

@@ -70,7 +70,7 @@ SRCDIRS+=
 
 # If you want to add arguments to the compile commandline, add them
 # to this variable:
-CFLAGS += -Oi
+CFLAGS += -O
 
 # If you want to add arguments to the assembly commandline, add them
 # to this variable:

apple2048/ui.c

@@ -33,7 +33,7 @@ void playSound(int8_t freq, int16_t duration)
         return;
 
     while (duration > 0) {
-        asm ("STA %w", 0xc030);
+        asm volatile ("STA %w", 0xc030);
         while (freq > 0) {
             freq--;
         }