This is very weird issue and also sad indeed.
I have heard of issues related to not good sound quality on certain audio chipsets but this one is first time for me. (then again i don't have much experience with onboard sound or GFX :ashamed
One friend moved to windows 7 but had some old motherboard and gets normal sound for the 2.1 speakers but no surround when connecting 5.1 speakers.
While i absolutely do not doubt your knowledge and understanding of this issue, are all the settings in windows and any other related applications set properly?
And can you please post those links you found on this issue?