One possibility is to get the audio dongle included in the 360's HDMI pack.
My understanding is that HDMI carries the audio with it, but if you're hooking it to a monitor, that'll be lost.
The dongle plugs into the regular 360 video port and has either RCA or optical audio outs.
http://www.amazon.com/Xbox-360-HDMI-AV-Cable/dp/B000UODATY/ref=sr_1_1?ie=UTF8&qid=1239130162&sr=1-1
Couldn't find an audio dongle-only package, but perhaps ebay?







