At the bottom of the remote there are four sound mode buttons.
While playing something from the TV, press the Movie button and you will be presented with this screen where you can choose/change the sound mode. The AVR will remember this and switch to it when playing this format again.
(The items on the menu will vary depending on what the source signal is.)
I would suggest Dolby PLIIx Cinema or DTS Neo:6Cinema as these are up mixers which will expand stereo to all available speakers e.g 5.1. (This is not the same a Multi Ch Stereo which just duplicates the two front channels in all speakers.)