Well, It's a bit difficult explain, but I played Monkey Island 2 and notified another great thing in SCUMM engine. To understand what I'm talking about, this game and a sound card is needed.
The game has midi-like music (I'm not really sure, what).
When entering into Largo's hotel room, music continues, but with extra instruments and a bit weird style! Leaving room music goes back to "normal", but it never stops playing! Extra instruments just "fade" away. How did they do that, and is something, that can be added to AGS,too?
iMUSE is the system the SCUMM engine had to do this (MI2 onwards, iirc)
http://imuse.mixnmojo.com/what.shtml
iirc again, ags has crossfading for music which i believe can get the same kind of result (but i've never tested it ;) )
This is the iMUSE system you are talking about, but we can do things like it, like adding a sound to an animation, like on a footstep sound, but that can only work for certain occasions.
did you also check out
PlaySoundEx <--more like what you want
PlayAmbientSound
PlaySound