I got a crash once for an invalid channel when falling to the
Water Elemental Plain from the Dark Side Skyroads, but couldn't
replicate it. So I've put in a warning for now
The existing structure didn't make sense, as Sound only handled
sound files, but Music handled both music and short FX decoding.
I've merged Sound & MUsic into a single Sound class, and moved
the music driver to their own file, renamed to SoundDriver