There is no official support for Apple beta iOS software, so I doubt Sonos hardware will look to support this new AirPlay feature until after Apple release their OS v18 publicly - simply because the beta software from Apple may alter before it’s officially released to the general public.

Rather than wasting time, many 3rd parties ignore Apple beta because Apple can make a late night pre-release change, breaking the 3rd party App that had been working with the beta before that late night change. Naturally, all Apple products have been updated along with the late night beta. Users then think that the 3rd parties are inept because the Apple Apps work. The 3rd parties may have been quietly working behind the scene and will release their update after they are sure that the Apple release is stable.

 Check phone Settings - Music - Dolby Atmos is set to Always On.

This is so weird. I was able to get it work yesterday by airplaying it to my ATV instead of directly to the Sonos speakers. Now it doesn’t work doing that. I don’t get it. I did change the settings to “Always on,” but that made no difference.

I believe this requires a system update from Sonos to support Atmos over AirPlay. I just updated my HomePod to 18.0 and I can stream Dolby Atmos music over AirPlay now. It falls back to Stereo when steaming to my Era 300s.

I don’t think (operative word there) that Apple allows the use AirPlay 2 to ‘stream’ Atmos to a non-Apple device. So you should be able to use AirPlay 2 to stream Atmos to your Apple TV, but not to your Sonos. The maximum stream that Apple allows AirPlay 2 to send to a non-Apple device is stereo. 

My point was that it is no longer playing Atmos even when Airplayed to the ATV. It automatically switches to stereo.

Sorry, I didn’t understand that properly. Does the Apple TV recognize it as an Atmos signal?

Sonos plays, for our purposes, anything it is handed by the TV set. If the Apple TV isn’t properly passing an Atmos signal via HDMI to the TV, the TV is unable to pass it to the Sonos for playback. 

To @Ken_Griffiths and @buzz ‘s point, though, it’s pretty rare for any third party company (both Sonos and the TV manufacturer) to spend any effort chasing Apple’s changes in a beta, they’d wait for a final release before working on anything. 

Updated iOS 18 today, Tidal AirPlay Atmos to ARC is working for me . Woot! Woot! Now if only it would work with my Denon AVR !!

It may be useful:
Sonos support answer to the question whether Air play 3 in Ipad iOS 18 works on Era 300 speakers and whether Dolby Atmos can be played:
"Sonos currently only supports AirPlay 1 and 2. It is possible that this feature will be available in the future."