I've had this problem for quite sometime and some of my friends with laptops experience the same problem. Sadly, there isn't a whole lot you can do. It really only happens when you leave your computer on for a longtime and usually stops working after you reset your computer or had it turned off for a sort period of time. Probably what the cause is something to do with your sound card (not sure what) and the part that forces sound to come through the audio jack is temporally disabled, probably due to too much heat inside the laptop but I could be wrong. Notice if you turn your computer off for awhile and turn it back on, the headphones (audio jack) works. If the sound does not work through the audio jack, turn your computer off for at least a half hour and turn it on and it should work. Sometimes I leave my laptop on all day for downloading movies and stuff and and I try to watch a movie at night but the audio jack doesn't work, so I turn it off and in the morning, it works.
