Sound driver RAM overhaul and general improvements

Changed inappropriate mentions of 'channels' to 'tracks'
Made the track RAM dynamic
Equated some track numbers and backup/restore sizes
Pointed out a bug in StopSoundAndMusic
Pointed out bug in Sound_PlaySpecial
Moved Constants to before Variables so the equates could be used to
create dynamic RAM
Added equate for the communication byte
Equated track data addresses
Aligned comments (tab = 8 spaces)
Added some missing comments

Most of the SMPS track addresses in Variables.asm are redundant, but I
don't know if I should get rid of them.

I couldn't equate  zTrackPlaybackControl(aN) because asm68k likes to
assemble that as 0(aN), which would break the (near)bit-perfection. The
AS branch shouldn't have this problem, though.