Audio via passthrough is nothing but a pain on BRAVIA, also depending on the FW version...
1. Using Plex from my HP Laptop to the Sony TV, everything plays in LPCM.
Is this correct?
Not quite correct. Plex determines that the TV can perform passthrough for Dolby Digital. The server is also capable of transcoding other multi-channel audio formats (like DTS) to Dolby Digital for passthrough.
Sony's current Oreo firmware pretends to support DTS too but fails to output it. So playing videos with DTS track might fail. In such cases, audio passthrough has to be disabled (either on system or app level) which will give you stereo PCM.
2. Playing back from a USB stick into the TV, audio is also LPCM
Is this correct?
Same as above, also depending on the app to play files from USB.
3. Playing the 4k disc of "The Matrix" on my Sony 4k player shows Dolby Audio on the soundbar and not Dolby Atmos when the correct audio is selected.
Again depends on the player app. Kodi should be capable of outputting DD+ based Atmos. True HD based Atmos cannot be output via ARC. This requires eARC (which your TV doesn't support). So tell us which player you use and determine what kind of Atmos your movie has (e.g. with MediaInfo).
