Bungie sound and music guru Marty The Elder posted today on the Marty Army site that soon we'll get more info on the second part of Halo 2's soundtrack:

I finished the Halo 2 Soundtrack Volume 2 today and listened to it (all 70 minutes worth) in one sitting. I liked it, and I hope you will too. I'll tell you more about it soon.

Can't wait.