Dynex 40" TosLink question

I am currently setting up a small home theater setup in my parents house (who are not the most advanced tech people by any means) and cannot for the life of me figure out their TV. They have a Dynex Dx-l40-10a which has a Toslink "out" on the back ...they would like it hooked up to their OLD Kenwood A/V receiver (VR-505). From the Hi-def cable box I have component cables to the Tv..I realize that I could go from the optical out on the cable box to the optical in on the A/V receiver..but I still would like to know why I can't use the optical out on the Tv.... I have all the connections done correctly but the Tv still outputs the volume through the Tv speakers and no the A/V receiver. Is there an option in the menu's of the Tv that I am overlooking or is the optical out on this Tv "just for show"?