I have a problem similar to the ones reported for many Vista users where my sound stopped working. I have tried all the same solutions including a system restore back to an earlier point, fixing the reg keys recommended in several reports, (although none were actually incorrect), tried disabling all enhancements on both devices. I also tried getting the original drivers from DELL.
I know for certain it is not a hardware problem because I have a dual boot set up on my computer both running Windows 7. The one works fine, no problems. The other doesn't work. The one difference is that the one that is still working was a clean install whereas the one that is not working was an upgrade from Vista, although it worked fine for quite a while.
I'm not sure what to do now because I've tried all suggestions that I could find in the Vista forum.