Fluctuating low/normal headphone volume on iMac 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?
I have this problem. The solution that woks for me is to open Audio MIDI Setup and mute-unmute channels in Output.