On-board Speaker Problem?

My speakers on my laptop have not been working for 5+ months now, but when I plug headphones in the headphone jack, I can hear sound, so I know it's not a soundcard problem. But if I leave music playing for about 1-1 1/2 hours I can get the sound back, so my speakers aren't blown. I don't know anything about the soundcard other than it has an IDT High Definition Audio CODEC.