src/audio.c:update_audio() drives the audio state machine. It is called from:

 - src/custom.c:DMACON()
 - src/custom.c:ADKCON()
 - src/custom.c:hsync_handler()
 - All audio register writes (handled in audio.c)

src/custom.c calls src/audio.c:update_audio() on hsync_handler:
 1. update_audio()
 2. fetch new sample for audio state machine if necessary



