just finished plugging in all the cables. there's a microphone channel, an audio feed from the presenter's computer, and a background music channel over iTunes/airport. they're merged at the mixing console, which sends the audio on to the video capture computer, the room P.A. and preview monitor headphones.

there's still some hum, but i'll get rid of that (maybe we need to ground the mixer?), and i should be able to fix the interlacing, too.