Which games in particular? ScummVM has supported the Mac instruments from Loom, Monkey Island 1, Monkey Island 2, and Fate of Atlantis for quite some time. A more accurate player for Loom, and a completely new player for Last Crusade appeared in the development version recently, so I'm hoping that means a Monkey Island 1 will follow. But I believe Day of the Tentacle and Sam & Max also had custom Mac instruments, and those aren't supported at this time.
I'm always hoping someone who has actually heard the real thing could check it out to see how accurate it is.
At least in the games that use iMUSE, each piece of music can appear in several forms, each tagged with an identifier to specify what hardware it's intended for. The DOS versions of Monkey Island 2 and Fate of Atlantis have music tagged as ADL (AdLib), ROL (MT-32), and even some SPK (PC speaker). Day of the Tentacle has ADL, ROL, and GMD (General MIDI). In Sam & Max, everything appears to be simply tagged as MIDI.
Based on some very quick testing, so feel free to correct me.