Can I plug a PC into one of the Onkyo HDMI ports? (TX-SR605)

Mike, I would agree. ANY device that either IS or emulates any hi-fi A/V device should work just fine assuming it doesn't deviate from the hardware standards of the 'real' hardware. Your Media PC is just another digital source as far as the receiver knows. Manuals don't mention everything because it is assumed that we know we can substitute compatible devices among the various in/outputs.

However, my own practice is to run every signal directly to the best device that enhances or uses it. NO VIDEO to my AVR (it goes directly to my monitor); NO HIGH-END AUDIO to the TV (it goes directly to my receiver). NO EXCESSIVE CABLES. NO PROBLEMS WITH UNNECESSARY CONNECTIONS. NO CORRUPTION OF ANY THE SIGNAL.

My PC's soundcard feeds both digital and analog audio inputs on my receiver so I have the flexibility of manipulating it in the analog domain for re-recording or just listening.