I'd take a look at your manual, is there some kind of on screen guide that you can pull up?
EDIT - I brought your receivers specs online and it says that the optical audio inputs are assignable, so there has to be a way to assign them or unassign them. If none of this works, there is an easier way, just plug the DVD player into the VCR inputs on your receiver instead of the DVD inputs, and then see if the sound works. If that doesnt work, then there is something wrong with the way your speakers are connected.
--Ben
EDIT - I brought your receivers specs online and it says that the optical audio inputs are assignable, so there has to be a way to assign them or unassign them. If none of this works, there is an easier way, just plug the DVD player into the VCR inputs on your receiver instead of the DVD inputs, and then see if the sound works. If that doesnt work, then there is something wrong with the way your speakers are connected.
--Ben