Even if I pick any MIDI device (including Fluidsynth, BASSMIDI, VirtualMIDISynth, etc.) in the music section of the Audio tab, the game still outputs music via Adlib. This is not the case when I play the game via DOSBOX with General MIDI as my music driver in setup.
I'm currently playing the game with the latest stable build (1.8) on Windows 8.1 64-bit.
This isn't a bug so much as missing functionality; the games were implemented based on reverse engineering the games, and we never had the time (or inclination) to figure out/implement other sound systems. It's a bit out of the way, but you can see the TODOs for the games/engine at http://wiki.scummvm.org/index.php/TsAGE/TODO
Ah. Fair enough, then.