@GardeningZen, The output will not be in sync.

This is unlikely to work well. You won’t be splitting the surround channels (or the .1) from the Arc, so they’ll all be downmixed into the Arc’s output. Also, given the different processing delays through Arc and the AVR, there are likely to be echo effects.

Yes, there’s risk with the combined sound fields from the Arc plus the ceiling speakers and sub powered by my receiver, but I think I can fine-tune things with the sound field settings and EQs. I just don’t feel like having to spend $699 for the Sonos Amp to replace my receiver.

I bought the Thenaudio ZONE-2 Processor and have it between my TV's eARC port and my soundbar's eARC port, and the audio (including Dolby Atmos) from Netflix, Amazon Prime Video, Spotify, and other apps on the TV is playing out the soundbar just fine. One caveat is that the video (for options settings) from the soundbar doesn't display on the TV as it would when I have my soundbar eARC directly connected to the TV eARC, but once I've configured the soundbar, I rarely need to change any settings, so this limitation is not a dealbreaker for me.

I also have one of the HDMI-IN ports on my receiver connected to Thenaudio ZONE-2 Processor, and wow, it actually works -- I have audio output from the TV playing on both my soundbar and receiver simultaneously! And neither audio sound stages seems to have any delay -- that is, no echo effects! I'll need to do more extensive testing with various video streams, but so far, I'm very impressed!

I ended up buying the ThenAudio Zone-2 Processor, and after installing it between my TV and my Sonos Arc along with my AV receiver, I can happily confirm that I’m getting audio (up to Dolby Atmos) playing out of the Arc as well as my AVR simultaneously and without delay!