Go to Tools > Options > Audio > Settings > DSP & output format. At the top right of the window under Channels, use the drop down list to select what you want. Select 5.1 then close the window (use the window close X) then click OK on the Options window and save changes. Remember that, with this option, all media will be converted to 5.1, even your stereo audio. If you don't want this, either create a Zone for audio or choose "source number of channels" in the option.
Nick.