In a perfect world:
The TV and soundbar perform a HDMI handshake, telling each other their capabilities.
The information is relayed to the Plex app by the TV. Plex then uses this information to decide how to handle each audio format.
With the Plex app configured with Passthrough = HDMI, the supported audio formats will be passed unchanged. Unsupported audio formats will be either (a) transcoded by Plex Media Server or (b) converted by the TV to a supported format.
The Plex Dashboard → Now Playing + Expanded View indicates what is happening with the server. It will show if Plex Media Server is transcoding the audio or video.
Unless the TV or soundbar provides feedback, there is no way to know if the TV is modifying the audio. There is no feedback loop to Plex.
You can deduce (mostly) what is happening by looking at the Dashboard, the TV specs, and the soundbar specs.
Example: Set Plex app to passthrough = HDMI and play a video with DTS audio, which is not supported by the soundbar.
If the Plex Dashboard shows the audio is NOT transcoded, then the TV is handling the process. Most likely it is being decoded and passed as PCM.
Example: The Plex app cannot passthrough TrueHD audio (blocked by Sony). It will be transcoded by Plex Media Server to another format.
Play a video with TrueHD audio and check the Dashboard. If it is transcoded to a format supported by the soundbar, then it is probably being passed through the TV to the soundbar in that format. If it is transcoded by PMS to a format not supported by the soundbar, then it is probably being decoded by the TV and passed as PCM.
There are times when things do not work perfectly or full information is not available. For example, when using an optical connection between a TV and soundbar. An optical connection has no handshake like HDMI. Therefore, it is not possible for the TV to know supported audio formats. If the soundbar did not support DTS, then there would be no audio when the TV passed it over the optical cable.
To handle these situations, Plex added Passthrough = Optical in the app settings. It makes the Plex app behave as if it was connected via optical. You can then select which codecs to passthrugh, AC3 and/or DTS.
Regarding DTS-HD MA…
DTS-HD MA is the lossy DTS 5.1 core + extensions to make it lossless. DTS:X works in the same fashion.
As mentioned in the post you linked, Plex would decode the core DTS 5.1 signal. The -HD MA or :X portion is discarded.
