A_Future_Pilot: Well the default uses MIDI...so it's whatever you happen to have on your system.
If you use a source port like DXX-Rebirth (There's another one...D2X-XL, but I honestly don't know much about it) then there are several AddOn music packs you can download, that are recordings of the MIDI music, on high-quality soundboards.
You can also use the Mac soundtrack with D1X Rebirth, it includes pretty good info within the options menu on how to set that up, and that rendition of the soundtrack is fantastic, though it's of course missing a lot of the tracks from the MIDI versions.
If you play with MIDI though, the sound system could make a lot of difference. :) I'd guess the original music was written for the Roland MT-32 sound card, but I'm sure you could get some really interesting results if you connect a polyphonic hardware synthesizer or keyboard through USB or MIDI and set up Windows to output MIDI commands through the MIDI out (I think the Roland (U)220 is supposed to be rather close to the original MT-32). Since most MIDI capable hardware follow the standardized instrument/preset orders drums and instruments should map to their corresponding sounds on the unit you connect. :)