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I first had this issue with my Apple TV 4K (gen 2), LG C1 TV and Beam (gen 2) when I initially set it up months ago, after weeks of playing with settings and some TV and Sonos software updates it started working correctly. However recently the issue reoccurred, after spending hours messing with the settings and thinking it was an LG TV issue I tired the settings below, specifically the last one fixed the issue for my setup. Hope this helps someone save hours of frustration. Please let me know in the comments if it helps with your setup.

LG TV Settings -> Audio -> Advanced -> Audio Format, for all 4 HDMI: Bitstream
LG TV Settings -> Audio -> Advanced -> Audio output to Direct Transfer/Passthrough
LG TV Settings -> Audio -> Advanced -> Share TV audio mode (TV Sound Mode Share) <— This fixed the issue for me

With the “Share TV audio mode” feature, do you now get “Multichannel PCM 5.1” audio from a Dolby Digital Plus 5.1 source on the Apple TV 4K?

I now get Dolby Digital 5.1 (in the app) from DD 5.1 and DD 5.1+ tracks (except Atmos DD 5.1+ tracks they display as Atmos) and Atmos from Netflix and Blu-ray also display correctly as Atmos. Prior to enabling this setting everything except Atmos was Multichannel PCM 7.1 and sounded really off.
 

Update: I’ve just tested some movies with DD TrueHD and DTS/DTS MA and all display as Dolby Digital 5.1 in the Sonos app and they sound perfect. However they must be being output as PCM due to the way Apple TV decodes the audio. Not really an issue it says DD 5.1 rather than PCM 5.1 etc in the app as the playback is now spot on. 

Interesting. So the TV is converting all LPCM audio from the Apple TV to Dolby Digital yet still playing Dolby Atmos content correctly. Do you notice any difference if you set the Apple TV to change the audio from LPCM to Dolby Digital rather than letting the TV do it?

What device are you using to play Blu-ray discs? Are you able to get Dolby TrueHD audio with the current TV settings? And what audio do you currently get from DTS-HD Master Audio encoded Blu-ray discs?

If I set the Apple TV to DD 5.1 and disable the TV sound mode share setting it shows the same DD 5.1 in the app for everything and you loose Atmos. It could be that the TV is now re-encoding everything as DD 5.1 or Atmos rather than the raw bitstream. Which although isn’t ideal if that is the case until there is a full fix from LG it sounds great and everything works as it should which is so much better than living with 7.1 PCM and sounds and voices coming from the rear speakers when it shouldn’t and a much quieter muffled centre channel etc.

Update: I’ve just tested some movies with DD TrueHD and DTS/DTS MA and all display as Dolby Digital 5.1 in the Sonos app and they sound perfect. However they must be being output as PCM due to the way Apple TV decodes the audio. Not really an issue it says DD 5.1 rather than PCM 5.1 etc in the app as the playback is now spot on. 

Are you watching actual Blu-ray discs on a Blu-ray player or Blu-ray rips on the Apple TV?

When you play Dolby Atmos audio either from a streaming app or from a Blu-ray disc or rip, is it truly Dolby Atmos audio? Do you hear any height channel effects? In the Sonos app, under System > About My System, what specific audio format does it display under the Beam information after “Audio In:”? Dolby Atmos, Dolby Atmos (DD+), or Dolby Atmos (TrueHD)?

Update: I’ve just tested some movies with DD TrueHD and DTS/DTS MA and all display as Dolby Digital 5.1 in the Sonos app and they sound perfect. However they must be being output as PCM due to the way Apple TV decodes the audio. Not really an issue it says DD 5.1 rather than PCM 5.1 etc in the app as the playback is now spot on. 

Are you watching actual Blu-ray discs on a Blu-ray player or Blu-ray rips on the Apple TV?

When you play Dolby Atmos audio either from a streaming app or from a Blu-ray disc or rip, is it truly Dolby Atmos audio? Do you hear any height channel effects? In the Sonos app, under System > About My System, what specific audio format does it display under the Beam information after “Audio In:”? Dolby Atmos, Dolby Atmos (DD+), or Dolby Atmos (TrueHD)?

Currently on this TV I’m testing everything from the Apple TV. I turned the TV sound mode setting off and re-tested DTS/TrueHD etc and it shows the inputs as PCM 7.1, all Blu-Rays and steaming services Atmos tracks show input as Dolby Atmos. Turning the TV setting on, everything shows the input as Dolby Digital 5.1, except Atmos tracks show as Atmos. I’ve never seen Atmos DD+ or TrueHD it always just says input Atmos regardless of the setting being on or off. I can also hear Atmos effects so it definitely working.

After reading more about this TV setting it seems like the TV is re-encoding the audio to DD 5.1 except Atmos tracks. The only other way I’ve found around this is to set the TV output mode to Auto instead of Pass through, which will also limit the ability to output TrueHD etc. So either approach will give similar results.

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