The data portion of the Descent CD is on the first track, the remaining tracks are CD-Audio.  Since these are not necessary to play the game, they have been compressed to MP3 format to save space.

Descent allows the use of any audio CD during gameplay; these MP3's can be used to make an audio CD or loaded directly into iTunes and used with D1X-Rebirth's Jukebox option.

Track List:
02 Primitive Rage.mp3(Main Menu)
03 Outerlimits.mp3(Briefing)
04 Close Call.mp3(Escape Sequence)
05 The Darkness Of Space.mp3(Credits)
06 Get It On.mp3
07 Fight.mp3
08 Death Lurks Beneath.mp3
09 C-4 Home Recipe.mp3
10 Escape.mp3
11 Backwards Time.mp3
12 Crazyfactory.mp3
13 Machine Gun.mp3
14 Insanity.mp3(Ending)

This package also contains a new playlist file (descent.m3u) for D1X-Rebirth as the builtin one DOES NOT work with iTunes 9 or later due to a change in folder structure introduced with it (if you have iTunes 8.2.1 or earlier, you don't need this new playlist file and can ignore the rest of this document as it's irrelevant). There are two ways to use this file. First, you can manually replace the original playlist file inside the D1X-Rebirth app bundle by right-clicking on the D1X-Rebirth app and selecting "Show Package Contents" then navigating to it's internal Resource folder and dropping in the new file replacing the old one. The second way is you can select the new file within the game while it's running by changing it within the "Jukebox Options: Path For Level Music" under the "Sound Effects & Music" category.
Also note that due to the same iTunes issue, you must manually select each of the non-level music tracks (02-05 & 14) in the same "Sound Effects & Music" section. The track list above can be used as a reference for determining which non-level music is for what purpose.