Why would sound suddenly stop working on Windows 7?

"Don't know much about any issues particular with Windows 7, but you can test if it's a hardware problem rather easily by booting up into an alternate OS installation. Assuming you don't have a dual-boot system, this can also be accomplished with a linux live CD that supports your laptop.

I personally use Knoppix for such things (mostly just because I happen to have the CD handy) but there's myriad other options available. If sound works on one operating system, but not under the other, then it's pretty much a software problem. If sound doesn't work with either, it's probably a hardware failure."