UPDATE!
I made it into a GOG mix.
These are all the games with soundtracks as of 09/28/10
http://www.gog.com/en/mix/games_with_soundtracks

Added:
Planescape: Torment

Sacred has a soundtrack included along the extras also.
Are any of these choral music or have this grandiose orchestral vibe? I like that kind of classical style music, specially if they have some loud chorus work.

Beyond Good & Evil
MDK2
Original War
Prince of Persia
Sacred Gold
1nsane
Alien Nations
Arx Fatalis
Ghost Master

Painkiller
Evil Genius
Earth 2150 Trilogy

Gorky 17
Gothic 1 & 2
HOMM 2 & 3

Neat idea! +1 for it!
Might and Magic 6 LE (for MM4+6)
(Port Royale 2)
(Patrician 3)- these two aren't really that much (some 10-30 seconds clips), but you may want to add them for completeness' sake

Disciples II Gold
Republic: The Revolution
Stronghold
Stronghold: Crusader

FlatOut
Jagged Alliance 2
Ishar Compilation

Here's an (incomplete) list I made earlier. I think it has all of them up to around when Republic: The Revolution came out.
1NSANE - In-game Soundtrack
Alien Nations - In-game Soundtrack
Beyond Good & Evil - Soundtrack
Darkstar One - In-game Soundtrack
Descent 1 & 2 - D1 In-game Soundtrack
D1 Best Soundtrack (MAC)
Descent 3 - In-game Soundtrack
Original Soundtrack
Disciples: Sacred Lands Gold - In-game Soundtrack
Disciples II Gold - In-game Soundtrack
Earth 2140 Trilogy - Soundtrack
Earth 2150 Trilogy - In-game Soundtrack
Earthworm Jim 1 + 2 - EWJ1 In-game Soundtrack
EWJ2 In-game Soundtrack
Earthworm Jim 3D - In-game Soundtrack
Evil Genius - In-game Soundtrack
Fallout - In-game Soundtrack
Original Soundtrack
Fallout 2 - Original Soundtrack
Fallout Tactics - Original Soundtrack
Flatout - Soundtrack
Freespace 2 - In-game Soundtrack
Ghost Master - Original Soundtrack
Gorky 17 - In-game Soundtrack
Gothic - Original Soundtrack
Gothic 2 Gold - Soundtrack
I.G.I. 2: Covert Strike - In-game Soundtrack
Ishar Compliation - Trilogy Soundtrack
Judge Dredd: Dredd vs Death - In-game Soundtrack
Knights & Merchants - In-game Soundtrack
Lionheart: Legacy of the Crusader - In-game Soundtrack
MDK - Original Soundtrack
Megarace 1 + 2 - Megarace 1 & 2 In-game Soundtrack
Messiah - In-game Soundtrack
Original Soundtrack
Neighbours from Hell Compilation - In-game Soundtrack
ObsCure - Soundtrack
Oddworld: Abe's Oddysee - MP3 Soundtrack
Original War - In-game Soundtrack
Painkiller Black Edition - Soundtrack
Patrician 3 - In-game Soundtrack
Port Royale 2 - In-game Soundtrack
Postal 2 Complete - Music to Go POSTAL By
Prince of Persia: The Sands of Time - Soundtrack
Pure Pinball 2.0 Redux - In-game Soundtrack
Redneck Rampage Collection - In-game Soundtrack
Rides Again - Soundtrack
Republic: The Revolution - Soundtrack
Rise of the Triad: Dark War - Soundtrack
Sacred Gold - Original Soundtrack
Underworld Soundtrack
Sacrifice - In-game Soundtrack
Screamer - Soundtrack
Seven Kingdoms - Soundtrack
Seven Kingdoms 2 - Soundtrack
Shogo: Mobile Armor Division - In-game Soundtrack
Silent Hunter 2 - In-game Tracks
Soldiers: Heroes of World War II - In-game Soundtrack
Stronghold - Soundtrack
Stronghold Crusader - Soundtrack
Tex Murphy: Overseer - In-game Soundtrack
Tex Murphy: The Pandora Directive - In-game Soundtrack
Tex Murphy: Under a Killing Moon - In-game Soundtrack
The Nations Gold Edition - In-game Soundtrack

Heh, you were doing it while we were making our contributions ;-).
Anyway, like I said, neat idea!

I hope this helps people, I will continuously update it.
If you're like me, and you want to go shopping, you hate actually clicking on the link to find out there's no soundtrack, so just browse this thread and then you'll never buy a game that doesn't come with some kind of audio file.
you audiophiles..

I'm sick of these bootleg-quality soundtracks - we need flacs.

Dayum thats a lot of soundtracks.

I know what you mean! :O
I look forward to it as a post-beta feature.