Hi, i have a problem getting sound from my pc to amp.

I have put my sky HD through the same route and it works, so i know i have it all set up okay.

Yamaha RXV467 HDMI3 with optical in to the second slot (AV3 i think its called) - as i said, sky hd worked well this way- plugged in to my soundcard (not inbuilt) on XPS420, changed in control panel (vista) sound, from speakers to digital output (SPDIF) and testing, doesnt work. tried to play music, dvd, bluray, .mvk, etc... and i get picture on my tv but no audio.

any ideas whats going wrong? Looking at the optical cable, its lighting up so i know its on and working well so the soundcard is okay, and it works with "speakers" (5.1 i got with the pc) although thats using the coloured cables.