I'm listening Halo 2: Original Soundtrack Volume 2, which (for me) is much better than Volume 1.
The orchestral pieces on Volume 2 are much more refined and better mixed (Vol. 1 was a marketing release to build hype for the game, and many of the pieces weren't complete yet).
Also, composer Marty O'Donnell arranged the pieces in suites, to better evoke the epic sense of the Halo story and universe -- and does it to pretty good effect.
Interestingly, in the interview, O'Donnell mentions he writes a lot more piano than ever makes it into the Halo games. But then the Halo 3 E3 teaser trailer came out ...
You can check out an interview about the Volume 2 soundtrack with O'Donnell music4games.com.
There are getting to be some genuinely good game musical scores out there -- Halo/Halo 2, Fable, Grim Fandango, American McGee's Alice, BLACK, Final Fantasy, etc.
Good stuff for folks who like good musical scores ...