External microphone with mid-2012 Macbook Air I cannot get any of my external microphones to work with my
mid-2012 MacBook Air (which runs Arch Linux).  Neither the
four-pin-jack headphone that comes with iPhones, nor the Beoplay
H9i (three pins).  It is clear in pavucontrol that only the
internal microphone is listened to.  Output works.  Any advise?
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.