After following advice on this forum, I used the D2X rebirth source port for Descent 2, added the .gog and .inst files to mount, and played. While I was glad to hear CD music, I realized the truth - the soundtrack isn't from the original game!
Using foobar2000, I played the tracks from the inst file and I can tell they're actually remixed tracks from the Definitive Collection, which presents a problem.
The image is actually missing certain tracks, like Glut, which was track 4 in the original CD. Track 4 in this image is Crawl, the credits music. Because the first level of Descent 2 plays track 4, the game is playing the wrong music!
And what about other tracks, like Haunted? They're not in this image.
Everybody including the GOG developers should take a look at this link and see the correct tracklist: http://www.descent2.com/goodies/music/d2_tracklist.shtml
If you can get the original music that play in the right levels, I would be grateful.

At the risk of breaking the rules, I'm posting links to the original track of level 1 and the one that plays for me.
Glut:
http://dl1.ffshrine.org/soundtracks/dl/580/8ef6ee/Descent%202/03_glut.mp3
Credits:
http://dl1.ffshrine.org/soundtracks/dl/580/9b80b6/Descent%202/02_credits.mp3
EDIT: The links don't work. Looks like you have to go to the site yourself.

Yep. I don't see how they could have done this, seeing how it utterly ruins the immersion the original music gave the game. Because there aren't enough tracks, the ordering the game reads is wrong.
It's been over ten years since I played Descent 2 from the original disk, but I remember what the first level sounded like, and it's in my second post.

(Apologies for the bump, but I feel the need to chime in right now.)
I just got an original Descent II disc at a thrift store yesterday, and you're right-the soundtrack IS different!
However, I still have another concern-if they changed the redbook soundtrack for Descent II, they might have done alterations to the first game's soundtrack as well. Do you know if they did change Descent I's redbook soundtrack or not for the Definitive Collection?

Descent 1 didn't have Redbook audio. In "Descent I and II: The Definitive Collection" which comprises the original Descent, Descent II, and Descent II expansion Vertigo, the only ones with Redbook audio are Descent II and Descent II: Vertigo.
I made a guide detailing how to create an Enhanced Hybrid Soundtrack from the 2 versions of Descent II's soundtrack
that has an extra 11 minutes of run time.

Actually, when the Vertigo series expansion came out, some of the tracks were moved onto the Vertigo CD. So all the tracks are there in the definitive collection. They're just not all on the main Descent II CD anymore. There are also some additional tracks that were introduced with D2 Vertigo.