I had the same problem with Logitech X-540s. I discovered that 7 rearranged the jacks on the back of the PC.
I have 6 audio ports on the back of my PC (like this):
1 2 3
4 5 6
In XP, 1 was Front Left/Right, 2 was Back Left/Right, 3 was Center/Subwoofer. But in 7, 5 was Front Left/Right. So now its:
1 2 3
4 5 6
Not sure why, but it works. I have full 5.1 now.
Hope this helps.