Plex picks the first audio track that matches your language settings. It does not pay attention to the default flag for audio tracks.
You could remux the file, placing the multi-channel audio track first and making sure the audio tracks have the correct language tag.
You could also try “tricking” Plex by playing with the language tag on the videos. For example, tag the multi-channel track with the desired language and the stereo track with a different language. Plex should then default to the multi-channel track.
Plex remembers track selections, so if you select the multi-channel track, Plex will remember that choice the next time you play the same file. If you want to reset to the default selection, then Plex Dance the file.
Remuxing / Retagging Tools: MKVToolNix; XMedia Recode, Subler (Mac)
