So...as it's been said several times, what we have here on GOG is not The Definitive Collection...however I have been able to track down everything that was in the Definitive Collection. Including all three versions of the original Red Book soundtracks.

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Basic Stuff
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Here's the Levels of the World for D1 (Last updated 5/18/2017)

Here are the Extra Missions that were on the D1 CD (Thanks to rs_tsyu)

Here's the DMB2

Here's Vertigo for D2
(To install it, copy everything into your main Descent 2 directory.)
(For Rebirth put d2x.mn2 and .hog in the missions directory, and put d2x-h.mvl, descent2.ham, descent2.hog, and hoard.ham in the data directory)

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Soundtracks: (Thanks to Nitroid, rs_tsyu, ThugsRook, and Ryusei)
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Here are the three versions of the Red Book audio as .zip archives.

Descent II Original Release/Destination Quartzon

Descent II The Infinite Abyss/The Definitive Collection

Descent II The Vertigo Series

And here are some AddOn packs for DXX-Rebirth from rs_tsyu and ThugsRook (If you use the Rebirth installer it will give you the option to download the Definitive Collection Redbook Audio, so you don't need to download it here):

Definitive Collection Soundtrack by rs_tsyu

Definitive Collection Soundtrack with Vertigo Soundtrack by rs_tsyu

AddOn Folder (This has all of the various soundtracks, as well as a hires texture AddOn for D1.)

To install an AddOn pack simply copy the .dxa file to DXX-Rebirth's main data directory. The game should automatically play from it when run. (Just so people know, these are zip files with a different extension, in case you want to extract them.)

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Additional things of interest:
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I'd also suggest checking out The Enemy Within, a complete level set for D2. If you thought Vertigo's puzzles were a pain, you ain't seen nothing yet ;)