That's an odd one alright! Initially when the betas were first out the 64bit 7000 build was a little touchy simply getting the correct Vista 64bit drivers
to work out. Since that time sound has never been a problem with the increased support for 7 being realized now over what was seen for Vista back in 2007!
I can't see where it would be any hardware problem since you have made changes and still run into this. The best idea on this would be if all this is being seen on one installation of 7 which may have gone bad on you. It points more at a Windows problem.