There was a policy that press was not allowed to videotape the behind closed doors demo of the first level of Halo 3 (Sierra 117). Through a loophole Snoopers from NeoGAF managed to grab audio from the demo. Tagged for fanboy’s only because it basically sounds like a bunch of gunshots and screaming. But you do get to hear some very pissed off Grunts, and Brutes, as well as the return of Sarge and what sounds to be the Arbiter.
Click the image for the link
Also, Halo.Bungie.org has a nice compiled list of links to the press write ups that you should check out.