Stating the obvious when I post, are the speaker cables correctly hooked up and have you turned the volume up?
The reason I ask, our cables like to pull out of the back of the receiver when moving the receiver around (new banana plugs are on order) and not paying attention to polarity, had changed the polarity on the cables when changing to different tips. D'oh! I did catch this error upon the standard, prior to power-up, visual system check. Also, I had forgotten to turn the volume up as a reason no sound was coming through the speakers. In this case, it was I who was getting all the dummy awards.
It sounds like you have your unit configured correctly as what you described is how I configured our receiver to accept optical input. If I might, a question regarding the computer, do you have the sound card configured to the optical digital out? IIRC, our soundcard requires this sort of reassignment configuring (optical or coax) if I want to use a separate headphone DAC. And do you have the sound volume on the computer turned all all the way up? I'm sure you know this, as all soundcard volume sliders should be set to max and let the receiver attenuate speaker volume.
Have you checked for dust in the ports or the ends of the optical cable? Did either end of the optical plug pull out of either the soundcard or the back of the receiver? Do you have a sharp kink in the optical cable as it sounds like you have a long run?
Hope the above ramblings helps give you ideas.