Hi. I have an HDMI output on my PC. In my case, I need to set in Windows if I want audio to go out the HDMI port or out through my soundcard. It doesn't do both. So, if you haven't tried this already, I'd suggest that you go to the following control panel:
Control Panel - Hardware and Sound - Sound
Select the "Playback" tab, and there should be a list of devices. One of those should be for your HDMI port. If so, set that as the default.
Once you get audio going out through the HDMI port on your computer, then you should be able to run it to the Onkyo as Mark first suggested.