Used by the routine at LOADING_TUNE.

Both voices' next notes are taken and their pitches looked up. Unless both are resting, the note is played for 18 passes of 256 turns of a loop that sends a byte to the speaker's port twice a turn, one for each voice. Each voice keeps its own copy of the port byte (voice one's in A', voice two's in A) and its own countdown (E, and L), and toggles the speaker bit in its copy when the countdown runs out; so each is a square wave at its own pitch, and the two sound together. The two ways round the loop take the same time, 96 T-states (the JR Z that never jumps and the NOPs pad the shorter), so neither voice's pitch depends on the other's.

Once both voices rest, it returns at once. That never ran in the sessions (the sounds page's runs reach it), and nor did the end of a note by way of the first DJNZ: which of the two ends a note depends only on voice two's countdown in the note's last turn.