Just to follow up. 5.1 definitely works, but there is also 2 channel stereo audio stream embedded as well, so the program/device you are using is defaulting to the stereo stream instead of 5.1. There are ways to strip it out if you are using a device without options to choose audio streams. Easiest way is probably to use handbrake to reencode it and drop the sac stero track. However you might lose some quality (though it does compress the file size). You could also look at tools to re-mix the file and also drop the audio stream. I think Paul Thurrott had a guide a few weeks back on ripping blurays that could help.
Alternatively, if you are on a computer use something like VLC.
Edit: If you want to Remux the file, you can follow Paul Thurrott's instructions
here, but make sure you select the appropriate streams to use.
How I got it to remux with only the stereo AAC stream w/ Shana Encoder
Right Click File Name > Video/Audio Stream Selection > 0:2(und): Audio: AAC.... ADD
Then make sure you remove the 0:1 stream above the 0:2 you have added in the lower right hand box