Like this?
From the looks of it, Plex already has a feature – however this seems only to kick-in if the audio is downmixed to stereo; some platforms seem to only apply this option if the audio is being transcoded. So I suppose this won’t help you if your client doesn’t tell Plex it’ll transfer your surround audio to a stereo-only device.
Based on the support articles, this is available on Plex for Android (mobile/TV), iOS/Apple TV, Playstation, Xbox as well as Plex Web
