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.