All great info; I'll have to check my Marantz for updates. Thanks everyone for keeping this thread active with findings. If I see any f/w updates, I'll hold off until I can successively test 15.2, the Apple HDMI cable (yet to receive), and finally AVR updates. Hopefully I'll get to it this weekend; been busy with work, holiday stuff, new girl, etc. I did do some music listening via Apple Music that including Dolby Atmos tracks after updating to 15.2 and didn't notice any issues. Also, I think the issue might be the same, but manifests itself in several ways; some of us experience the popping, some the muting, some both. It seems like something is losing its sync and we hear that as popping, but perhaps it gets so far out of whack that the audio drops as the AVR tries to reestablish the connection and start decoding again. On my setup, I actually experience both; sometimes 5 sec of popping, and sometimes several seconds of muting (when muting happens, I see my Marantz drops decoding altogether, and then goes down to stereo, then to multi-ch, and finally back to Atmos before audio resumes, almost like it has to sync back up). Will be very interesting if the new HDMI cable has any affect as I'm running what I thought were 8k HDMI 2.1 cables (tried several already). But, going to go in steps so that if anything does indeed fix the issue, I'll hopefully know what part of the chain was failing.
