Music 4 Games has interviewed Marty "The Elder" O'Donnell about the Halo 2 soundtrack. Here's what he had to say about what's different this time around, under the hood:

As far as the game engine itself, we've completely revised and upgraded every aspect. The AI, physics, graphics, and audio have all been reworked for Halo2. We just couldn't leave well enough alone. The music engine gives me all the features I had before, but now with surround sound mixing and interactive markers that are imbedded in the tracks themselves. I can define how the markers respond to gameplay information.

Sounds like it's going to be great. Thanks to Louis Wu at HBO for the heads-up.