I suspect it's something to do with the HD Audio manager - will experiment in the next couple days (have a 5.1 Logi audio setup waiting on system to be moved to final location - stain on desk is drying as I type) and get back to you, say - Monday...
I have a 5.1 set of Logitech G51's. I tried for an hour, and the rear speakers would always show up as side speakers. What I finally did was call it a 7.1 system and disable the the side speakers.
Also you want to check where the audio cables are plugged into the back of the PC. If the computer is on, Gigabyte boards software asks what kind of speaker you just plugged in. I messed them up when I first hooked up the cables because I thought only the color counted. I don't know why you would want to do this, but you can change the setup. After all that, it ignores the fact you called them rear speakers.
Do you use the software volume control or the hardware volume control with a preset software volume?
Oddly enough, after a little fiddling, I arrived at the same point - set it to 7.1 w/no 'side' speakers... Also, the more options (like Dolby) that you enable, the more likely that you'll have audio 'stutter' problems in WMC - disable everything possible, and it sounds fine! Another oddity that I've noticed is that in WMC under Vista x86, the sound seems 'biased' to the front speaker triplet, as I'd prefer; under x64, they seem 'biased' toward the rear, so I leave it selected to two channel - can always adjust for actual 5.1/7.1 encoded on DVDs.
Here is my suggestion. Return any Gigabyte motherboard. The company builds crap and doesn't stand behind the product. I have a brand new board. Channel C won't work so I can't use triple channel memory, and the rear speakers won't work either...