There is no setting in the app for that. The article you are reading is for the Plex Media Player, which is a desktop app. It is not the generic name for all Plex clients.
Your symptom sounds like you are playing back surround sound audio on a non-surround setup. If so, there is an option in the app to turn off passthrough. There is also a global Android TV audio setting to not support surround sound, turn that off.
I have tried the disable passthrough option and it didn’t help.
I am using a 2.1 sony HT-CT380 soundbar with a Sony KDL 65W850C TV.
Those options are disabled when using the sound bar.
I have used the voice up on the soundbar but its still not the best.
I was hoping there would be something I could configure in plex that would transcode the 5.1 to 2.1 on playback.
To use your soundbar, you will need to turn back on surround sound support in Android TV and the passthrough option in the Plex app.
If your device correctly reports to the Plex app that it only supports 2.1, Plex will transcode to that. If this is not working, I will need to see the logs from the Android app and from your PMS after you recreate this issue so I can see what is going on.