Marty appeared on the HBO forum to clarify some things regarding the audio effects and music we heard in the GameStock '01 Halo movie.

The weapon fire sounds, the impact sounds, the reload, and the yeehaw all came from the game engine. The music came from a CD I was playing in the back of the hall while Joe hit the marks we planned. The four pieces of music were composed and arranged for the gamestock demo. The final sound hardware, programming, and content is still (surprise) being worked on.

This information isn't wholly surprising, as some of the audio at the E3 demonstration last year was controlled manually as well.