This is definitely a thermal problem, on my acer TM2480 sound chip is getting hot after a while when comp is on and then pluging the jack gives no sound. I managed to cool the chip by turning the comp into standby, and, normally, we need to wait til it got coooled. Then I discovered that the headphone plug is getting hot after a while being plugged, so figured out a solution:
Just after turning it to standby i started to suck with my mouth through the jacks, i done this for several times, until when i plugged the phones, the plug stayed cool, so that the internal things in nbook should be cool too, and that's it, after wakin up, the sound is back!
It works even when the comp was workin for many hours, just make sure to cool the chip inside, you can use a vacuum cleaner, it cools very fast, and you dont have to use refrigerator
By the way, there's another solution... a simple external sound card plugged in usb, the 2 ch models are quite cheap, about 20$, and it should realy do for us
