Downloadable community music
The FreeSpace Music Restoration Project (FSMRP) was a now-defunct effort to retrieve the uncompressed versions of the FS1 and FS2 musical scores. What music the FSMRP did collect can be downloaded here. The music shipped with the FreeSpace games was in a compressed format, and the lossless FLAC files were stored on tape drives. Some of the information on the tape drives have been recovered by Volition and released to the community; the rest has been either unreleased (mainly because either FSMRP or Volition didn't bother to share) or corrupted. Hiring a data recovery specialist to retrieve this data has not been a priority for the company.
Music in user-made campaigns
Beyond the Red Line has a completely original soundtrack composed by MaX. The BTRL soundtrack has been widely praised by the FreeSpace community, and has been used in more than one FSPort campaign by Orpheus.
Blue Planet has more new music than any FreeSpace mod, save BtRL, and has been widely praised for its soundtrack. Included is music from Final Fantasy, Dune, and various internet music sites such as Freeplay Music. Most notably, it makes use of the excellent Second Reality (S3M format) track, upgraded version of the Inferno Release 1 soundtrack named "Syphon", which has never been used in official Inferno campaigns.
Transcend is one of the most "cinematic" of all FS campaigns, and it's been praised for its Betrayal.wav, the campaign's only "original" (eg taken from a game other than FS2) music track.
The Procyon Insurgency and BrShivans.rar contained some music tracks taken from other games. PI also used some FSMRP music (see above); in fact, CP5670 (the campaign designer) was a member of the FSMRP.