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.
Merge pull request #2185 from unknownbrackets/sas-minor

Correct envelope scaling in sas mixing