Apple tv 4k switch to pcm stereo randomly

Again, it seems like the ATV4K is not hearing the TV communicate the status of the Sonos. Could be a timing issue during power on, or the Sonos goes to "sleep" after a time-out interval so the ATV4K reverts to stereo. The ATV4K tries to adapt to changing system attributes whereas other devices like your xbox may just take it on faith that the device it was configured for is ready-to-go.

To test this, unplug the Sonos and note that the ATV4K reports your TV's audio features. Restore the Sonos and (perhaps after a while) the ATV4K reports Atmos available. It'll certainly see the Sonos if the TV and Sonos are powered properly on when the ATV4K is rebooted.

• If your Sonos reports PCM, that means uncompressed audio, not necessarily stereo. This is as intended for Auto format and Atmos enabled.
• If the source stream has stereo audio, then Apple TV will output stereo audio.
• Set the Apple TV audio output to ‘TV Speakers’ and your TV to audio passthrough. Selecting the audio system as output would not use the HDMI connection, and trigger a (temporary) AirPlay or Bluetooth connection for audio, which is limited to stereo.

Please tell us what type of audio content is playing when this happens. Dolby 5.1? Dolby Atmos? A multichannel stream may switch to stereo if the ATV4K thinks the Sonos Arc isn't there...

Your TV's eARC connection to the Sonos Arc is critical because the ATV4K needs to see the Sonos' capabilities to deliver the correct multichannel audio stream. Verify all the HDMI settings on the TV and the Sonos to make sure that all the enhanced / eARC / CEC / automatic functions are fully enabled on both devices.