when I played it, the audio was downconverted (from 6 ch to 2 ch) and I don't see any option to correct that.
It is possible that rather than being down-converted, the audio stream that was played was actually the stereo track. For the BD at least MC will just play the default track. You could hover over the "Audio Path" information box (right most button under the information window) to see if MC is playing a 6 channel or 2 channel track.
If you try right-click > Stacks > Advanced > Particles > Auto-Create particles from DVD or BD (this might be slightly off, as I don't have MC in front of me at the moment), you might find that it creates a new "particle" that contains the surround mix. I believe the Wiki has some information on particles, but you can think of them as a "virtual file" or a playlist, loosely speaking.
If you can get this to work, it should give you more flexibility creating views down the line.