I've done that with DOSbox. Playing Monkey Island through DOSbox -> Qsynth sound a lot better than ScummVM -> FluidSynth, due to the different reverb settings (Qsynth is a FluidSynth frontend).

The FluidSynth reverb, while not as good as the old Live!/Audigy hardware reverbs (which were quite good), can still sound decent with the right settings. I have no idea how to change this in ScummVM, but I have found the following FluidSynth settings to sound much better: Reverb: Room: 0.61 Damp: 0...