Do you have a one connect box? Where are you located? Geography seems to play a large part in how the Arc interacts with your TV. I live in Canada and have a 2020 Q80T, Arc, Sub Gen 3 and a pair of One SL’s. My system works flawlessly to date. Get Atmos content where available from all the streaming services using my ATV 4K. I have upgraded all my cables to HDMI 2.1 and use Cat 6e Ethernet cables throughout. My home is wired with Cat6e as well. 

I do not have a one connect box.  I am located in Arizona.I am connected with a HDMI 2.0 cable.  I ordered a HDMI 2.1 cable.  I am using wireless.  I have no issues with the wireless for music nor did I have issues with my old TV connected to the Arc HDMI.  My router is an AX3000 router.  I am replacing the cable but do not have high hopes that will fix the issue.

Try to power cycle the tv and Soundbar to see if the problem continues. Unplug and reconnect the HDMI cable connecting the two devices. If the problem persistsyou should call Sonos Tech Support for assistance. 

With the Connect Box I remember reading the problem goes away if you use ARC instead of eARC. Does this work for you - though more of a work around then a solution?

I have the exact same tv as the OP and have no issues using eArc. His setup should be working. 

Good to hear that. Can you share your TV's sound settings?

Arc plugged into eArc HDMI port. TV sound to “Receiver”. In Expert Settings HDMI eArc Mode set to “Auto”.  Digital Output Audio Format set to “Auto”. Digital Output Audio delay set to “0”, Atmos “enabled”. As stated above I am using new HDMI 2.1 cables throughout as well. 

Do you have the External Device Manager set to Anynet+?  I purchased an HDMI 2.1 cable and that did not change anything.  I have everything set just like you.  If there aren’t any other suggestions then I will give Sonus a call.

Anynet+ is on

As it should be: without it the TV doesn't know there is an Arc connected.

For anyone with the same issue.  I had to power everything off then turn the TV on and then the Sonus.  That worked for about a week, then it started happening again.  It kept switching between the eArc HDMI and the TV.  I searched on Samsung’s site and found they just released a new firmware.  I upgraded the firmware and it has now worked for over 2 weeks now.  The only issue that I now have is when I switch to a channel it will not have any sound.  I then have to switch to another channel then come back and the sound works for the original channel.  Not perfect but we are improving.

I just purchased a new Samsung Q90T and Sonos Arc with Sub. I have been having problems with eARC not working.  I have spoken to Samsung who told me they can’t help troubleshoot Sonos and their own soundbar, Samsung, works just fine. I have also called Sonos and have had 3 different techs over many hours on the phone try and fix the problem without success. The last tech told me to connect to optical via adapter, which I believe limits performance.  Does anyone know if there is a fix coming from Sonos.  If not I will be forced to return the Arc or switch the tv to a different brand. Any help or advice would be appreciated.  

I had issues, then upgraded the firmware.  I was able to get everything to work for a while until I moved and had more issues.  I then turned of eARC and had it working with HDMI without eARC.  No issues for over 2 weeks.  Keeping my fingers crossed.

same issue twice my samsung 65q95t …. Sonos Arc is plugged with sonos cable to hdmi arc port but sound cutoff every time !