It would be great to have the possibility to let the music be played by external hardware.The music of Doom was originally composed on a Roland Sound Canvas device and it would be a really nice touch to let owners of this hardware experience the music on the real thing.
While on other OS it is possible to solve this issue externally (for example on Windows XP using the midi mapper), on current OS
X versions there is no way to do this AFAIK. Either the midi data is send OS X coremidi or it is not, using coreaudio the midi files are always being
played by the internal soft synth.
Sending the midi data out to the core midi facility also has the advantage of letting OS X users use a 3rd party software synth (for example fluidsynth).
There are also ongoing problems with the internal soft synth in OS X Lion leaving Lion users with screwed up music tracks.
For reference, ScummVM and Dosbox both support this requested feature.
Windows build already supports fluidsynth, portmidi, opl2 and sdl_mixer+timidity for rendering midi files. Unfortunately PrBoom-Plus has no MAC developer, that's because you have what you have. Fluidsynth, portmidi and opl2 stuff was coded by Nicholai Main, so probably you can try to contact with him (natt on doomworld.forum)