Are the connections to the Sonos Arc (presumably from your switch/extractor) an HDMI-ARC connection or an eARC connection? - you need an eARC connection for Dolby TrueHD (uncompressed) audio whereas the FireTV 4K Max can use an HDMI-ARC connection, as the DD+ Atmos audio from that device is uncompressed. So perhaps that’s what is happening here - it’s just not clear from your post what type of HDMI ports are in use in the setup.

Ahh! I in fact DO NOT have an eARC HDMI output on My TV. It is confusing to say the least. Can I assume that if My Blu Ray player can output Dolby Digital plus content than, that I can get Uncompressed Dolby Atmos sound? Sounds to me like you are saying the Blu Ray player can only deliver TrueHD audio via an eARC HDMI connection. My point is how does the fire stick get around this hurdle yet the BD payer does not? Thanks

The Blu-Ray disc you’re playing is going to be encoded in Dolby TrueHD ..it would need transcoding to DD+ (Atmos). I don’t know of a player that does that, personally speaking. I’ve only ever seen them transcode to DD 5.1, or of course fallback to PCM stereo.

Somewhat irrelevant here, but with DTS HQ/X uncompressed audio discs (not supported by Sonos) users with eARC tend to set their players to transcode the audio to multichannel Linear PCM audio and those that have ARC only ports on their TV will transcode the audio to compressed DTS 5.1 (which is now supported by Sonos) but there’s no support for Atmos here either. 

What should work for you is to perhaps bypass your TV using an audio extractor with eARC support, such as the HDFury Arcana, or Feintech VAX04101A etc. I use the latter one myself.

Anyhow I hope that info. assists.

THANKS I was just about to order the HDFury Arcana when your post appeared. I looked into the Feinteck device but read some less than favourable reviews? Are you happy with it?

Yes, I’m really happy with the Feintech device, it works great here and have heard lots of good things about the Arcana too. I went for the Feintech device as it has a built-in HDMI switch (3 connected devices). 

Hi Ken. I have since ordered the U9 Audio extractor and with the updates it seems to work reasonably well. My issue however is that I am still unable to get TRUE HD Dolby Atmos audio and on a new Blu ray disk, 7.1 DTS Master audio to my Sonos Arc. The U9 Supports eArc output to the Sonos and I have set the Blu ray player to output (Auto) bitstream data to the port but no dice. (defaults to DTS 5.1 audio? Any suggestions?

U9 extractor? - I’m not sure which one that is. Which movie disc are you playing that is outputting Dolby True HD audio soundtrack and which Blue-ray player - make/model?

What are the audio out settings on the player too?
 

Bladerunner 2049 Dolby Atmos 7.1 On my Sony UBPX700/CA Blu-ray Disc Player.

https://www.amazon.com/gp/product/B097NXXDG6 and also have Avatar the way of the water which is encoded with DTS 7.1 Master Audio Track?

Presumably that disc doesn’t have the DD TrueHD audio track then? - note the only DTS codec that Sonos soundbars support is DTS 5.1 compressed audio. They do not support DTS HD Master Audio tracks

Both Blade Runner 2049 and Avatar: The Way of the Water have Dolby TrueHD/Dolby Atmos audio tracks on the Blu-ray discs.

Be sure to set your Blu-ray player to these settings:

Digital Audio Output: Auto

BD Secondary Audio: Off

Audio DRC: Off

FYI… for DTS-encoded discs, set Digital Audio Output to PCM. This will allow you to get Multichannel PCM audio from all DTS audio tracks.

If the "Avatar: The way of the water" has Dolby Digital True HD (Atmos) track as @GuitarSuperstar mentions, then personally speaking, I would just go with that codec.

Correction: The 4K UHD disc of Avatar includes a Dolby Atmos audio track, BUT the standard Blu-ray only includes a DTS-HD Master Audio track. If you have the standard Blu-ray, set the Digital Audio Output setting on the Blu-ray player to PCM to get Multichannel PCM audio.

Both the 4K UHD and standard Blu-ray of Blade Runner 2049 include the Dolby Atmos audio track.

It appears to only offer the 7.1 DTS Master audio track (At least on my Disk) 

Update! I now get the following and this is Great. Thanks for all the tips guys