I had to register to say I have this exact same problem. Through temporary fixes by either sleeping then waking the computer up, or more quickly just disabling the playback speakers then reenabling them, it works for 30 seconds to 5 minutes, but lessens with each fix it seems. The only permanent fix for me has been using my Turtle Beach headset, though I would think that any headphones would work.
I tried updating my drivers, I uninstalled the device and reinstalled it, and I even ended up formatting my computer, reinstalling Windows 7 and reinstalling all the factory drivers from the CDs they sent me originally with my laptop. My only conclusion is that its a hardware issue with the internal speakers themselves. Unfortunately, Im not willing to part with my laptop for 2-4 weeks while CyberPower fixes it, so I guess I'm just going to get used to the headphones unless someone has another possible solution?