A bit of followup…
Some background
There are two forms of Atmos audio used in home theaters: TrueHD + Atmos and Dolby Digital Plus + Atmos (EAC3 + Atmos). TrueHD + Atmos is found on HD/UHD Blu-ray discs. EAC3 + Atmos is used by the streaming services such as Netflix and Amazon Prime.
Blu-ray discs with TrueHD + Atmos audio also have a Dolby Digital (AC3) version of the same audio track. Discs with dts-HD MA or dts:X audio also have a dts version of the same audio track.
To determine which audio formats are in your media, examine the file with MediaInfo. It will display the format of each audio track in the file.
Plex clients do not indicate if Atmos information is present. They identify audio tracks as either TrueHD or EAC3 whether or not Atmos information is included. In addition, dts:X audio is identified as dts-HD MA.
HDMI-ARC, used to carry audio from the TV to the audio equipment, is bandwidth limited. It cannot carry lossless formats such as TrueHD or dts-HD MA. It can carry AC3, EAC3, EAC3 + Atmos, and dts with 5.1 channels or less. It can also carry PCM 2.0.
HDMI-eARC, an updated standard, can carry lossless audio such as TrueHD & dts-HD MA. Both the TV and attached audio equipment must support HDMI-eARC. If not, the connection will fallback to HDMI-ARC.
HDMI-eARC side note: TV manufacturers do not currently permit SmartTV apps to pass lossless audio via HDMI-eARC. So, a Plex app on an eARC enabled TV cannot pass TrueHD/dts-HD MA audio to attached eARC capable audio equipment. Devices attached to the TV’s HDMI inputs can pass lossless audio via eARC. So, a Shield attached to an eARC enabled TV can pass TrueHD/dts-HD MA audio to attached eARC capable audio equipment.
Your Setup
Your TV supports HDMI-ARC. Therefore, it cannot pass TrueHD + Atmos to your soundbar. It can pass EAC3 + Atmos. The same is true for any devices connected to HDMI ports on your TV.
To play TrueHD + Atmos audio via the Shield, it must be connected directly to your Soundbar. The soundbar will play the audio and pass the video to your TV.
The Plex client app on the Shield should be configured for Passthrough = HDMI (Advanced section). This tells the Shield to pass all supported audio formats to the soundbar.
In the Shield settings, set Display & Sound -> Advanced Sound Settings -> Available Formats = Auto (should be default setting). This lets the Shield and soundbar communicate to determine supported audio formats.
There may be applicable settings on the Samsung soundbar as well. Check the owners manuals and applicable setup menus (I don’t have such a soundbar, so can’t tell you what, if any, settings to check).
