Seems it's strange but it's the solution I tried and it worked when my PC (with Windows 7 Ultimate x64) cannot play any sounds. Of course I tested my speakers, my headphone, re-plugged speakers and headphone, reinstalled sound card driver, but all of them didn't help.
But it doesn't mean you need to go to sleep, then wake up. It means you need to put your PC into Sleep mode then when you wake it up, the sound will back.
HTH if you have same problem.