Put your finger near in the front of headphone socket (about 1/2cm), are you see the red light ? if no, its may occur by driver. try reinstalling the driver or updating the driver ... if yes. it happen because headphone socket(jack) support SPDIF (digital out) its a mechanical problems (my 4810t had the same problems) i just push the spdif sensor in headphone socket with a needle. sometimes its working. sometimes i jush punch the socket :LOL. better take your laptop to acer service center.