Halo 3: ODST extends the New Mombasa conflict, which first started in Halo 2. Aside from doing your homework on Firefight mode and the campaign, you may also want to prepare your ears for the audio assault they're about to undertake by listening to the Halo 3: ODSTOST.
AOL Radio has the goods on its Video Game Scores station, though you'll have to put up with non-ODST tracks between the good stuff. We just wish Sgt. Johnson was deejaying the station, but beggars can't be choosers, can they?