Fluctuating low/normal headphone volume on iMac

Question

Sound was working normally on this pretty new iMac for a few weeks, listening to YouTube when this strange behaviour started:

There is a little pop sound, then suddenly headphone audio goes very quiet (maybe 10% normal volume). There's no change in the indicated volume level (ie, the Mac thinks it's outputting normal volume).

Maybe 30-60 seconds later, another pop and it's back to normal volume.

And then repeat...

It affects the whole system (ie, sound effects), not just Chrome.

Any ideas?

Answer 1

I have this problem. The solution that woks for me is to open Audio MIDI Setup and mute-unmute channels in Output.

Answer 2

Just for completeness, when someone else looks at this question:

First, look into the headphone jack for a red light or a blinking red light -> if so: ifixit.com

Then check if the issue is caused by the headphone buttons, whick are faulty by trying another pair of headphones.

And if nothing else helps, try the same as Steve and reset the PRAM: Apple KB