My Sonos Arc just got delivered today and I’m having the exact same trouble. Sony 900h, Series X, Sonos Arc. Weird thing for me is the Arc works great with just the 900h through apps. Audio only drops on Series X in 120hz mode.

Same issue on the x950c or costcos version. Weird thing is that it was working just fine 2 day ago but when I booted it up this morning there was no sound until I changed it back to 60hz.

Same problem here. Sony X950H.

Also

I have just purchases a full sonos set, Arc/Sub/2x ones, also a new Sony X950H, the problem is, i am trying to get Atmos or at least DD 5.1 plus, with no success, either using the built in Android on Sony TV (also the audio cuts out, but i think that is a known Sony issue) and the new Chromecast with Google TV. any ideas?  

This is happening for me also at 120 Hz

Same issue with x900H for me. 

This is a known issue with the Sony, they are working on it.

Not a Sonos or Xbox issue.

Something I tried..weird but works. I tried everything then i stumbled across a nifty workaround. I also have issues with 4k 120 and atmos only with my new console on my x900h.  I synced my headphones and the drop outs stopped. Ask me why and I could not tell you.  Your headphones have to be turned on and synced with the xbox in order for it to work. Like I said extremely weird but it stopped dropping out. Turn your headphones down put them to the side and enjoy the atmos experience lol. I really hope they fix this soon.

I have the tv below the 900 model for Sony with the full Sonos set up and twice today my arc lost connection to the app after powering down the Xbox series x I’m fairly new to the Sonos world. Both times the app said I lost connection to the internet, second time around I still had full sound but no access app wise, had to wind up redoing the whole set up process twice today, if anyone has any tips to remedy when this happens instead of having to do a full system set up it would it would be greatly appreciated after looking around on the forums I believe it was the arc itself that lost the connection somehow. Any tips for the right way to fix this instead of starting over from scratch?

It sounds like a local network issue.

Here are my suggestions...

• You could try changing your routers 2.4ghz Wifi Channel - use ‘fixed’ Channel 1, 6 or 11 (non-overlapping channels) and see which one works best for you...ensure the 2.4ghz band and 5ghz band have different SSID names.

…....

• Or better still, try running your system on SonosNet, with one Sonos device wired to the main router (not a bonded surround device). 
• Set your SonosNet channel so that it is at least 5 channels away from your routers 2.4ghz channel to prevent overlap. Always fix the channels, rather than allowing the router to auto-select them on startup and avoid channels in use by your neighbours..
• Ensure your WiFi credentials are NOT stored in the Sonos App “Settings/System/Network/Wireless Setup” (They are not required when running a system on SonosNet).
• Add all your Sonos IP addresses to your routers DHCP Reservation Table. This is optional, but highly recommended.
• Make sure that your Sonos devices are at least 3 feet away from any other wireless devices, including the main router.
• Where practicable, connect any other non-Sonos devices to the 5ghz Wifi band. 
• Switch off any guest WiFi radios when they are not in use/required.
• If using WiFi access points, or repeaters, with the same SSID as the main router, then ensure they operate on the same 'fixed' WiFi channels as the main router.

This Sonos Support document is also worth keeping in mind... 

• WiFi Interference

Hope those few things assist and prove to be useful.

Awesome thank you for this Info, I’m in a fortunate position where I can hardwire the Arc to my router without showing cables so that is what I will do next if it happens again, I really appreciate that!

Anyone find a fix for this? Still finding the audio cuts out constantly with the series x but only on 120 Hz mode. Really disappointing as I bought the full surround set up to go with the series x.

If it’s just happening with TV audio, then perhaps look at the other connected TV peripherals to see if they are taking hdmi-cec focus. I would start by removing the other devices connected to the TV HDMI ports and see if the issue goes away, if so, then reconnect each device until you find the ‘culprit’. Then switch off it’s CEC control, or use a cec-less adapter.

That wont help, its a problem with this specific TV @120Hz due (I think) to VRR. Sony need to fix it, if they can. Hopefully they are not using the same defective Panasonic hdmi chipset that Denon/Marantz are broken by.

Ah right, thanks controlav.

I did try this to no avail, thanks for the suggestion. Behaviour is identical even when it's only the arc and Xbox hooked into the tv.