1up's interview with Marty O' Donnel
...EGM: One new audio feature for Halo 2 we're really looking forward to is the location based voice chat online, where you hear other people talking from the direction they are at, [with] the relative volume change...
O'Donnell: It's not like this is a brand-new thing; I think Rainbow Six did some stuff like that. We're really working on what we call proximity voice [and] how that works in the game. Because Xbox Live supports voice-over IP, we're always really interested in trying to make multiplayer games more fun for co-op or teams when you have your buddies online. We feel like there have been a lot of people who play a certain way -- they'll get four other guys together and they'll get a conference call going when they're playing a PC game. We would like to give those kinds of capabilities to people who are together in teams or squads or clans, and also at the same time, allow voice that's happening as though you're hearing it in the game. You can have what seems like radio, coming from your headset, and what seems like that guy to your left actually talking.