Just going to ask, because I've never done this...
If audio out of the PC is connected via optical, does it come in at full line level to the AVR, or would the volume control in Windows still impact that level?
I was just thinking that if the volume on the PC itself is turned down from 100 to say, 20, then that would have a significant impact on the audio if volume control is supported over the optical output on the PC. In which case, the audio out of the PC does need to be adjusted up to about 80 or 90 to ensure that it is being received at a proper level into the AVR.
Going to HDMI won't fix this either if the main volume level is set too low on the PC itself.