i had the same problem. Through research found that it was a sensor in the headphone jack. I rebooted a few times; plugged headphones in and out a few times. Blew in compressed air. Then thought of seeing if some kind soul had written an app to take more control of sounds in a mac... found this https://staticz.com/soundcontrol/ ; installed the trial version and it fixed it inimmediately. Full disclosure: I'm an ordinary Joe user and in no way associated with the software company. i will probably wind up paying the $10 for the full version if cannot find a free alternative ;-