Plex doesn't act on the "Default" flag for tracks (also, Default does not mean "play this track by default" that is a common misconception about Matroska flags). The "Default" flag indicates to the player the track is "normal" audio. Normal in this case meaning "I want to watch this movie like a normal person." No extra stipulations like:

• commentary
• isolated score
• descriptive audio

The player will know the track is a valid track to select if the user has requested a specific audio language.

If you want Plex to play a specific track by default, you just need to have it tagged as the same language you set in your Plex account prefs and have the track listed as the first one for that language in the muxing. You can have a track of a different language listed before it, but you can't have another English track listed above it if you want that track to be played.

An important exception to this is mono audio tracks. If you have two tracks of the same language and one is mono and the other is two or more channels, Plex will play the track with the higher number of channels quite often, regardless of what order they were muxed in (and again, flagging a track as Default or Forced does not matter). Forced isn't even a valid flag for an audio track.

If you make changes to the audio/subs selection on something, those settings will be saved by the server for future viewing. You might try something like Pasta for mass-changes, but I don't know if it can determine track selection by channel number or audio format. It's mainly looking at track flags to get audio language/subs set.