Sadly, my reading of the manual is that the Pioneer AVR can't do what you want. Pioneer's source selection menus have fixed input signal associations and don't seem to include the option of setting audio input priorities. If an HDMI video signal is present on one of its inputs, I don't see any way to tell the 9130 to use the corresponding optical audio instead. (HDMI audio is transmitted in the vertical blanking interval of the HDMI video signal.)
There's a possible workaround, though:
Some models of receivers will continue to display the most recent video source when you select an audio-only input. If your PC has a coaxial (RCA) digital audio output, you could connect it to the AVR's CD RCA digital input. First, select the AVR's video input where you've plugged in the DVI/HDMI cable so the video is being displayed, then select the CD input. Does that work for you?
(I don't have a Pioneer, so I can't test this workaround. I know it works on my Marantz, though: I've used it to provide background music for an otherwise silent slide show.)
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped