The new xbox update is supposed to support h264 in an MP4 container, but the AAC audio must be downmixed to 2 Channel Stereo. Whipped up a tiny script to do this for you - I'm leaving the demuxing of the 5.1/6CH .AAC up to you, as I have no idea what type of container you have. All the script asks for is that the audio file has an .AAC extension. Long file names and spaces in file names are supported.

* This uses the free Nero Audio MP4 audio encoder. It will automatically take you to the download site if it doesn't find the Nero EXE files.

aac_6ch_to_mp4_2ch.zip