Correct envelope scaling in sas mixing

Sound effects seem a bit loud, but this matches what JPCSP does and I've added comments based on my understanding of it.  It makes sense.

This makes all the sounds that never get muted when the envelope hits 0 go away.  Hurray.

Thanks to help from @i1x.

Fixes Twin Brave, Tales of the World, etc.  Does not fix Kingdom Hearts of Final Fantasy Tactics.

-[Unknown]