When you enter a mission or are watcing for baddies to jump at you, some mood music can be heard.
Something simple, like so:
MP3: https://docs.google.com/file/d/0B-G-3jTfhPIaZC02dUlrX0hPRGc/edit?usp=sharing
Original XM Music module (uses only 3 tracks, spent 10 minutes only on it)
XM: https://docs.google.com/file/d/0B-G-3jTfhPIaQ3B5VW00UEFHcjg/edit?usp=sharing