Rewrote the sound group functionality.  There is now a first-class
object for each sound group with a lot more built-in code.

This may have finally fixed the bug where we get multiple VO playing
at the same time.  I hope.  However, it may have introduced a new bug
where VO doesn't play.  I've left a fair amount of debug logging in.
If VO is missing (or if you see any error messages), please save the
log and send it to me.