The folk at Battleground: Halo scored an interview with Marty O'Donnel during E3. Here's a quick snippet:

Battleground: Halo: Someone claims that there was a new piece of master music in this movie, will this song be released as well?

Marty O'Donnell: There is quite a bit of newly recorded music in the movie. Much of it is based on the original theme, but there are also some new sections. It's a film score, so it's there to enhance the emotional message that we're trying to get across in any given scene. Also, the main them itself has sort of become a musical signature for Halo which is a good thing from my point of view. We're still deciding how and when to release the movie and the soundtrack to the public, stay tuned.

Marty is the audio god whose magic has been included in Bungie games since Myth: The Fallen Lords. Total Audio has also done work on titles as famous as Riven.