Dell laptops use a really flaky thru hole method to attach the headphone & microphone jacks. They are attached with the bare minimum of solder and thus develop dry joints with minimal use. I repaired my brothers Dell by fully dismantling and desoldering the headphone jack, which was broken beyond repair. As it's a custom part I couldn't find a replacement so I desoldered the stereo mic jack and swapped it over. This is a NON-TRIVIAL repair and took me almost 2 hrs to complete.
Another option is to simply replace the headphone jack with a $2 USB audio device:
http://www.dealextreme.com/details.dx/sku.22475