It depends on your video card, but it's very possible you could connect your video card internally to a digital output on your motherboard or sound card to send sound via HDMI.
I have an Nvidia GTX 285 and it has an internal input right on the card for a 5.1 line ... which I connected to the motherboard's internal SPDIF audio output. Everything's sent to the TV in one simple cable.
The strange thing is ... the GTX 285 has no HDMI port, just dual DVI. And DVI isn't supposed to send audio. But the DVI->HDMI adaptor that shipped with the card actually passes audio through as well.
Might be worth investigating if you have a newer PC if it's capable of mixing the digital audio in with the HDMI cable for a nice and simple solution. My PC is in another room and so I just use a bluetooth keyboard/mouse and a 50ft slim HDMI cable from Monoprice to my Panasonic 58V10 plasma. Works like a charm.