The soundtrack is available for the other two games. Originally Descent 2 had it's soundtrack in Redbook audio format so it would've been easy to get hold of from the game disc.
Is it possible to rip the soundtrack from the gamefiles somehow?

I thought I read in another post that you could mount the .gog files in Daemon Tools or equivalent and the audio tracks were included in the image, but now I can't find the post...

I mounted the .inst file in the Descent 2 folder, and used foobar2000 to read from the virtual drive. From there, you can rip the tracks.
However, the tracks in the .inst file are from the Definitive Collection, and are not from the original game!