I have seen this request and complaint many, many times over. A movie, say “The Terminator,” has mono audio as its default in audio track 1, but in addition it has stereo commentary in audio track 3. Plex will choose the stereo commentary in track 3 as the default audio track. Plex also defies (beyond ignoring) any metadata flags used to try to force it to do otherwise.
I have numerous titles for which this causes problems, and would love to see Plex finally fix it.
I have personally seen and confirmed with Plex for the web, Plex for Android, and Plex for Desktop. A user has reported Plex for Roku as exhibiting the same behavior.
How would this behavior be different on various clients? As far as I know, Plex does not have different audio selection settings based on client. It is based on account level settings and since the information is saved in the DB, that is then the default for all clients. And more specific, the account level settings are actually for language preferences and not audio codec/channel settings, to clarify I am not saying those exist.
For example, if I change the audio track from the default of “English Dolby Stereo” (my default track for the majority of my media files) to “English Dolby 5.1 Surround” (what I prefer because I have a surround sound system), that is the track that is selected when I go to any other client (Plex for Android on my phone or on my Shield TV for instance).
I fail to see how this is client specific, so I am asking this question (and providing my reasoning for questioning your statement, especially since you say “…this behavior is definitely different in various client types.”) without any ill intentions but to understand your thought process.