I just sumbled across this the other day. Check out the site for the Halo 3 Official Sountrack, here. Click on the “Extras” button and watch the video. In the video it mentions that the music in Halo 3 is more or less scored on the fly. From my understanding that means each time you play the game, depending on the actions occuring, the music will be different. I thought that was particularly interesting, as I never would have guessed that was the case. Mind you, I’ve only had the opportunity to play through the Campaign mode once, when a friend came over with the game, but even then I wouldn’t have guessed the music was being scored as we played.
The last time I have seen that much innovation in the audio of a game was back in the day of Trespasser.