Used by the routines at UPDATE_WIZARD, UPDATE_SERF and UPDATE_KNIGHT.

Called every frame by all three characters' handlers -- UPDATE_KNIGHT, UPDATE_WIZARD and UPDATE_SERF -- so walking sounds the same whoever is doing it. A counter at $5E2F advances on every call and its bottom two bits do all the work: bit 0 decides whether to make a sound at all, so only every other call does, and bit 1 chooses which of two notes.

The two are four cycles each, of half-period $60 and $40 -- which measure at about 1360 Hz and 2005 Hz. They alternate, so a walking character produces low, high, low, high rather than one repeated click. That is the whole footstep: two tones and a counter.