It could be the actual jack or maybe a bad connection between the front panel and the motherboard.
First thing would be, with the power off, to open the case and reseat the cable from the front panel. Most newer Dell's have one cable with all the front panel functions that connect to the motherboard. Reseat - unplug cable then plug back in making sure it's fully and correctly seated.
If the connector reseat does not help try "cleaning" the contacts on the mic jack by pluging in and out rapidly several times and also turning the plug in the jack several times. That MAY help the mechanical connection.
If neither helps, and since it's relatively new and still in warranty, best to call Dell support.
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