I agree, though the latter can contribute to the former, they aren't necessarily the same.
I'll add the mandatory Deus Ex nomination, particularly the Main Theme and UNATCO Theme. Although seeing that Deus Ex: Invisible War has an entry at slot #88, I suspect I may not need to...
Edit: Oh, and of course Halo's main theme, along with several other solid Martin O'Donnell contributions. Another game that I've left up at the menu for the sake of the music.
Yeah, I mean Halo is a perfect example. The main theme falls into the latter, but there are so many tracks that work only as ambient noise that perfectly encapsulates the specific moment in the particular level.
Link to video.
Also? Halo 1 is a goddamn perfect game and I'm really annoyed that nobody voted for it in the Game of the Year thread.