I would suggest using OGG instead of MP3 for music packs, mainly because Rebirth seems to be having problems with MP3 playback right now. (Also, the current music packs use OGG files, so using that format for CD music as well will make things more consistent.)
I already have music packs made from my own rips of The Definitive Collection; PM me if you want them. I'll also give you a fixed version of temple2.hog, as well as the extra Descent 1 missions from The Definitive Collection that were not included with Levels of the World.
Now for the Descent 2 track order:
Main menu: Track 2 (that is, the first audio track), then track 3
Credits: Track 3
Levels: All tracks after track 3. The track that plays first depends on the level number and the number of tracks on the CD--level 1 plays track 4, level 2 plays track 5, etc. Once the last track is reached (for example, level 10 and track 13 in the original version), the next level will play track 4.