Why does my sound sometimes stop working in Vista?

I've had a similar problem, described by Rafael Rivera here. I fixed the problem by installing newer drivers for my onboard audio chipset; try checking if you have the most up-to-date drivers (maybe check the manufacturer homepage). At least the link will better explain what is probably happening.

I had similar issues - it turned out to be a faulty audio driver. Find what on-board audio device you have and download the latest driver.If it's any consolation, in Windows 7, all you'll have to do is right-click the speaker icon, and Windows will solve the problem for you (i.e. restart the Windows Audio Service and run through a series of steps to correct the issue).