I don't know exactly what you're defining as "remixes," Zane, but the tracks listed in the Descent music section of PD really aren't (or at least, not in the strictest definition of the word). Asrale and Aenn Sidhe Priest's tracks are re-creations of the original MIDIs, while Valin's are re-recordings of the MIDIs with a legacy FM-compatible sound card. Overclocked ReMix does have a single
Descent II remix, from that game's credits track. I also have 3 or 4 remixes of original Descent tracks lying around here somewhere; I've been meaning to find out where I snagged them for some time now, so I can put a news post up about then.
Dark Hunter, the original Descent did use all 22 of those MIDI tracks; you might be thinking of the game's demo, which only included the first 7 levels. The
Descent Archives have the majority of the Playstation Descent tracks available; I believe that any tracks they don't have on that page were just versions of the D2 redbook tracks. As for buying any of the Descent games, Amazon.com has plenty of fairly cheap copies, both new and used. I believe that a few Fry's Electronics stores also have copies of Descent 3 lying around.