RE: Inspiron 17 5748 does not recognize external microphone
Oddly, the laptop had the same problems with the Logitech h540 headset, which is a USB headset.
A usb audio device is a separate audio system from the integrated Realtek audio, because it does not use the Realtek codec. It has its own DAC and ADC, which are controlled by the Windows usb audio driver, which is part of Windows and not a separate download. When you plug in a usb headset, the system might not automatically switch to it. If you still have it, plug it in and go to the Recording devices tab. There should be a new listing for a usb device. Make it the default recording device. If it does not appear in the list of devices then it is probably defective, or something wrong with the usb port. Also make sure that the recording program you are using to test the mics is set to use the usb mic.