When my Apple TV is set to Dolby Atmos but movie is Dolby 5.1 it only plays Multichannel in why? The Apple TV is connected to a new Denon S760H

The content determines the audio output, unless capped off by device settings. 5.1 content will pay as 5.1. Stereo content will play as stereo. Mono content will play as mono.

Multichannel can be 5.1 - just losslessly decompressed from Dolby’s format.

It works as intended.

It is designed like this on purpose; it’s not a bug. Even if you see no DD symbol on your AVR, because it is no longer in that format. There’s nothing wrong with multichannel audio 5.1, exactly the same quality, it’s just decoded on a different device.

If wish to express your concerns to Apple, then you may use the feedback form: Feedback - Apple TV - Apple.

I have the same problem. When you select auto, it plays ATMOS fine, but if the source is 5.1 , then Apple TV outputs PCM STEREO!! you have to change the output in setting to 5.1 to get surround sound.

When streaming content with 5.1 audio from online streaming services, Apple TV with the ‘Auto’ audio format setting

will output PCM, but as Multichannel (i.e. surround sound). PCM means uncompressed, not necessarily stereo.

Without any info on your setup, I can’t comment on what happens for you. It is working as intended for other users.

Verify that the C9 is configured for eARC (there is an on|off setting for eARC): LG OLED C9 Series eARC support - Sonos Community

eARC unlocks more bandwidth, so that PCM Multichannel can become a valid option.

This AVSforum post talks about audio issues with the LG C9 and in combination with certain receivers, notably Denon, and users with the same issues as you.

Verify that your TV and receiver both have up-to-date firmware, as that is how issues get fixed by the manufacturers.

It’s not for me. It’s sending PCM stereo on auto when the source is 5.1 (Apple TV, Netflix, Plex).

Apple TV connected to LG c9, which is connected through ARC to DENON 6300.

LG HDMI set to pass through - which is working since Atmos sources work fine.

It is. Also, ATMOS is working.