The latest in GameSpy's series of interviews with Bungie is with Marty "The Elder" O'Donnell, about the sounds and music in Halo 2. Marty always gets asked a lot about music, but this article also talks a bit about the game's sound effects:
Hitting a cabbage with a metal pipe works wonders. Also this time we have doubled the sampling rate, and have stereo/surround sound files that are crisp, juicy and crunchy. Plus, we killed a lot of grunts while recording.
Poor lil' guys...