Who is doing the sounds and music for Halo?

Marty O'Donnell and Mike Salvatori, formerly known as Total Audio in Chicago, did the sounds and music for Halo.

Marty O'Donnell accompanied Bungie to Washington state; Mike Salvatori retained the Total Audio name and remained in Chicago.

Halo's musical score is supposed to be dynamic, reacting to events within the context of the game world.