It could be several different things. But I suggest that your next step is to buy a can of electronic contact cleaner (Radio Shack carries one) and, using the little "straw", spritz some in the headphone jack. Then jack in the head phones once or twice. Then do it again. And THEN try out the headphones.... a set that you've tried on something else; so you're certain they work.
Danielle
April 2008