Hello,
I have a very similar problem (Pavillion g7, Windows 7) - with the headphones plugged in sometimes the sound comes through the headphones, and then just randomly (with the phones still plugged in) sound just starts coming out of the speakers. This doesn't seem to have anything to do with which programs are running, it just happens randomly, e.g. halfway through listing to music, several times during a Skype call.
I've tried all the suggestions listed above but hasn't fixed it yet. Any suggestions would be gratefully appreciated plese :)
Many thanks!