Hi @Chucky K
Grouping for TV audio via HDMI is not supported - it may work, but rarely flawlessly.
Normally, when playing music, the room in charge of the group (listed first) will pre-buffer the stream (even if it’s from Line In) prior to playing and sharing it on the network for other rooms. This allows the grouped rooms to be told at what exact moment they should play data they already have (due to the buffer) and so everything plays together and in sync.
When you group to play an HDMI/Optical source, the Sonos device getting the stream from the TV will play it immediately, this time to stay in sync with the TV picture. Thus, there is no pre-buffering done before transmitting on the network and grouped rooms are at the mercy of network performance from moment to moment. Reducing traffic from other devices will help matters, but by how much is impossible for me to say. I would expect an ethernet connection for both rooms to give the best results.
What to take away from of all this is that if you can play a music stream that you’d normally play on Apple TV from the Sonos app instead, you will get much better results doing so. As for video content, telling the Apple TV to AirPlay audio to the two rooms may yield better results than HDMI as the Apple TV will delay the picture enough that buffering of the audio can be performed. Do the buffering at the source, essentially.
I hope this helps but even if not, you will at least have a better understanding of what’s happening and why.