Used by the routine at upd_144_to_149_152_to_157.

Plays on every other frame of the walk cycle. It alternates between a fixed pitch and one set by the walker's height, by bit 1 of the complemented frame counter, which gives the tick-tock of two feet; the number of waves comes from X and Y. Sabreman's steps, sound_walk_step, use the frame counter uncomplemented, so his two pitches alternate the other way round.