http://support.xbox.com/support/en/us/xbox360/gamesandmedia/movies/videofaq/viewvideoplaybackfaq.aspx
Q: Does the Xbox 360 console support 5.1 channel AAC?
A: No. Only 2-channel AAC is supported. If you want to play a 5.1-channel video on your Xbox 360 console, you will need to encode it to WMV with WMAPro 5.1 audio.