Fix recent ADSR regressions

Fixes #5383, probably/hopefully also #5379.

We can't really key off when we set playing = false.  If it's not playing, it won't step, and if it doesn't step, it'll never end.

Although, I'm not sure, maybe it instead forces the height to zero for an ended voice.... I don't think it will matter since the voice will not play either way until reset to attack.

-[Unknown]