I'm posting to see if it helps anyone. I was having audio problems with my Lumia 535 and headsets. Now I'm having a similar problem with my 640. There would either be dropouts or the audio would just stop coming through. I tried swapping out different headphones and the ones without the microphone on it worked fine, while the ones with the mic had all sorts of problems. I'm not sure if it's a hardware problem with the phones or a software problem.
I just took one of my problem headphones, put an adapter on it (so the end piece now has two black rings on the tip instead of three) and it's working fine now.
I just took one of my problem headphones, put an adapter on it (so the end piece now has two black rings on the tip instead of three) and it's working fine now.