I run my system through the HDMI in my video card to an AV receiver, and I'm getting 7.1 audio just fine. Some times when I get a driver update, I'll have to reenable 7.1 audio though, which seems to have been made more difficult with this update. I'm not sure if you're experiencing a similar issue.
Try right-clicking the speaker icon in the taskbar, select Speaker Setup -> Advanced setup. Click on your audio device, then hit the Configure button. On the next screen, pick 5.1 or whatever you want to use, then hit Test to change the signal and make sure it's working.