hifi engine

Problem to get sound from my pc through NAD L73 receiver!?

I´m new with these A/V receivers. I get sound from TV but not from my PC. My systems is a ASUS 5E3 Pro moterboard with external soundcard, HDTV and a digitalbox. The PC is connected with HDMI to TV.
I tried to connect the PC with optical Toslink to TV but the sound only came out through the TV,s speakers. Can someone advice me how to get out thef sound through my NAD receiver to my speakers?