The headphone port appears to be a headphone/mic combo port on that model. I would start by installing the latest audio drivers from Dell. Goto the Dell support site, enter the Service Tag# from the white sticker on the laptop, and you will see a list of drivers available.
This is assuming you are using a headset with a standard 3.5mm jack and the Dell audio port (not USB).
This is assuming you are using a headset with a standard 3.5mm jack and the Dell audio port (not USB).