External microphone with mid-2012 Macbook Air
More than likely, you are using a non-Apple headphone so the Mic pin is in the wrong location on the TRRS jack

My advice is to avoid using the built in a jack and use a quality USB microphone or at least a quality DAC that accepts standard headphone input  I’ve personally gotten better audio quality going the USB route (no pops or hums) versus the built in connector.