Errrm, well the first thing that springs to mind is, the new Sherlock Holmes film (a game of shadows) has not been released to DVD or blu-ray yet, so if it's that film you have then it's likely not an official copy!
Other than that, the first question is, is it a all in one system or is the blu ray player separate to the surround system, if it's separate how have you got it connected?
And no, it won't automatically select the right format unless you have set it on automatic, if it's set on a 2 channel mode then it will only play 2 channels regardless of what you put into it.