The audio track MUST be being broadcast in the current selected surround sound mode to be outputted in 5.1. You are probably watching a film on the DVD, and most films have at least a 5.1 sound track. Has your system got a selectable surround sound mode? If so, try cycling through the modes. Most music is not recorded in surround sound either.
So basically it depends on what you are watching as to weather it will be outputted in 5.1 / 7.1
Hope this helps!
K.