How can I force 5.1 output over HDMI on my Geforce 660 in Windows 8?

I have my computer's Geforce 660 connected via HDMI to my TV, which is connected via optical out to a 5.1 soundbar + surrounds. I can force my PS3 to output 5.1, and this is passed through to the soundbar successfully. However, on my Windows 8 machine, I believe the drivers communicate with the TV which reports only 2 channel capability (possibly because of its internal speakers) so that's what my computer sends it.

I can't find a way for the TV to report 5.1 capability to my computer, so instead I'd like to force 5.1 out of my video card. It seems like there are ways to do this on previous versions of Windows by installing special drivers but I can't find anything for Windows 8.

Here's the screen where I was hoping to see 5.1:

Might these instructions on creating an EDID driver help? I know they're for ATI cards, but I don't think that matters.