This appears to be a bug/issue with Vista/Windows 7 Realtek drivers. I can't comment on 32 bit, but I know this is happening with Windows 7 64-bit for sure. I did not have this issue with XP 32-bit.
As people have said before, playing 2-channel audio with Speaker Fill ON sounds distorted in every speaker except the front 2 channels. If you play 5.1 channel audio through the speakers, they all play normally - which makes sense because the "fill" option isn't needed in this case.
Like I said, I did not have this issue at all in XP. Speaker Fill would play audio correctly in all speakers. I'm hoping Realtek realizes and fixes this issue, however, to be fair, this might be an Vista/Windows 7 issue. Either way, something has definitely changed in the way Speaker Fill works from XP to Vista/Windows 7.
I saw that 3D Sound Back program, but I didn't think it would work in Windows 7. I'll give it a try since the previous user says it's working ok.