Hi @Morgano4u.

Thanks for reaching out and welcome to the community!

I appreciate your detailed post here outlining your concern, let me help and try to figure this out.

Kindly start or play the TV audio playback using the specific content or media in question, then submit a diagnostic report through the Sonos app, and reply with the confirmation number. I'd start by reviewing your system and the status of the devices, it will also help in identifying what's possibly causing the issue.

If Arc is receiving an Atmos signal, it will be displayed within the Sonos App on both the "Now Playing" screen, and within the settings portion of our app: Settings > System > About my System > Sonos System Info > Audio In

For reference, you may check our support article that outlines the main factors that determine if your Sonos Arc can play Dolby Atmos audio.

I’m looking forward to assisting you further from here.

If you have any other questions or run into any issues,  please do not hesitate to reach out,  we are always here to help.

@Morgano4u , some questions to try and figure this out:

1. Does this happen only for TrueHD+Atmos files? (I.e., TrueHD no Atmos is okay, DD+ Atmos is okay, etc.)?
2.  What playback program do you use: Plex or something else?
3. Is your playback program transcoding the audio, or passes as bitsream? 
4. Is TrueHD playback from Apple TV 4k fine (it will strip away the Atmos, though).
5. What happens when you use your TV’s native apps?
6. Are all devices connected to different HDMI ports on the TV (i.e., are there no switches involved in your setup)? 

I am asking all these to try and actually troubleshoot your issue. I have a suspicion that the culprit isn’t Sonos Arc. I doubt the diagnostics suggested by @Annazel S will shed much light on your problem (but I may be wrong). 

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Hope this helps.

I have only seen this behaviour with Atmos (TrueHD) streams. I do not have any non-Atmos TrueHD content as far as I know.

I use the Plex player app on my Shield Pro 2019 box with a TCL R625 over eARC

No transcoding in the Plex app for audio or video

Never was able to get TrueHD passthrough with ATV4K, read that it does not support it. Hence I bought the Shield that does.

Native Roku Plex app on TCL will not passthrough TrueHD.

All devies on single ports yes. Using Monoprice HDMI 2.1 cables, also used the Sonos cable for the Arc at first then swapped it when I noticed that downgrade from Atmos (TrueHD) to Atmos (DD+).

A very annoying bug as the sound quality is quite different with lossless audio.

Actually I have the TCL 55R635 with eARC.

Seems like an eARC issue considering DD+ is usually sent over ARC. Roku 9.4 update will address this maybe. Any news with my diag @Annazel S?

Thanks to @oriboaz for helping out.

Hi @Morgano4u.

Thanks for your responses and update.

Upon checking on the diagnostic report, there’s no issue detected on the system and everything seems to be running smoothly except that the home theater codec format or the audio signal the Arc receives is Dolby Digital Plus (DD+).

If  the TV supports Dolby TrueHD as well as the content playing, and the Arc is connected to the eARC port of the TV, there’s only one factor left to verify if Dolby Atmos TrueHD signal is passing through without an issue and that is the source device. Getting Dolby Atmos audio can also depend on the physical source device playing the content.

Sonos speaker can only play what is handed to it. If the source device isn’t feeding the TV the correct data, or the TV isn’t feeding the correct data to the Sonos, then that will really cause the problem. 

Yes, you may proceed with the Roku update and hopefully that will solve it but if not, it’s best if you can reach out with Roku support for further assistance provided that we are certain that the TV supports Dolby Atmos and not re-encoding the audio to DD+.

The tricky thing about Dolby Atmos is that, for it to work, every ingredient in your home theater setup has to support Atmos.

Hopefully, that helps and if you have any other questions, please feel free to let us know, we’re always here to help.

In any case this is a handshake issue. I just don’t know how to troubleshoot it. I presume updates will fix my issue considering my setup is very common and other users will have the same problem where TrueHD works for a stream and later that day revert to DD+ or DD 7.1 until you cycle the Arc or pull the HDMI from it and plug it back in where the handshake brings back TrueHD.

Hi @Morgano4u.

Thanks for your response and for your patience while working through this.

If you are still running into trouble after, it would be best to reach out to Sonos Support so we can take a closer look and continue working with you in real-time to expedite the process.

You can give us a call and work with a technician live, they'll start by checking your system health and go from there. They have resources available for advanced troubleshooting, and the tools our agents can use would be very useful in this case, there can be an advantage if they need to remote into the controller to see what's going on.

Please let us know if you have any other questions or concerns, we are always here to help.