…additional info: The problem disappears when I use headphones… but also disappears when I use VLC or YouTube etc. So what's that?

Yes. I have the same problem. MacBook Pro (16-inch, 2021) Monterey 12.2.1. Until recently I've been using my Focusrite audio interface for everything but I noticed when I unplugged it that all Apple-created audio [so Apple TV, QuickTime videos etc] has terrible crackling and popping on when using the native audio engine. If I play the same QuickTime videos through VLC they're fine – so it's not a hardware problem. As far as I can see I can't reinstall Quicktime without reinstalling the whole OS which I am absolutely not going to do. I guess the only thing to do is wait until Apple release a new software update which will hopefully overwrite the broken component. Assuming they notice it's broken of course. Has anyone at Apple ever responded to any of these threads?