I feel your pain. I have been investigating this exact same problem (although I have a different ASR board than you) and after reading a few dozen posts scattered about the net, it's not possible on our boards. And to that end, I'm calling bee-ess on ASR* and how they advertise their board specs. While true that the Realtek codecs may support 7.1, the MOBOs won't and they should just leave that bit off.
Here's the spec for my board from the ASR site: 7.1 CH HD Audio with Content Protection (Realtek ALC898 Audio Codec), Supports THX TruStudio™
Funny thing...TruStudio is included in the initial install and there is an option to upgrade to TruStudio Pro which, like a dummy, I did. The problem is...nothing works in surround. TruStudio Pro doesn't even provide the option for 5.1 or 7.1 because of the output limitations from the MOBO. So, I wasted $25 on something that was doomed to fail. The only way to get proper sound is with a sound card and HDMI, as has been pointed out.
Lastly, here's a thread I stumbled across which describes this problem in detail (pay special attention to scorrpio's posts in the thread):
EggXpert - No option to output 5.1?
*ASR isn't the only board maker doing this.
Bookmarks