Hello. Are you plugging the headphones into the headphone jack socket, or are they usb headphones that plug into the usb port?
If they are normal analog headphones plugged into the headphone jack they should work no matter if they register as speakers or headphones.
My advice is to
1. Test the headphones in another device to make sure they work.
2. Re-install the audio driver in the laptop. Doing that will reset the default settings of the driver and the headphones should work. I would give you a link to your audio driver but to do that I would need to know the name of your operating system and the model name of your laptop. The Inspiron 15 models are:
Inspiron 1545, 1546, 1564, M5010, N5010, M5030, N5030, N5040, N5050, 3520, 3521, 3537, and 7537.