movie additional language soundtrack naming

I have an mov file of a movie (Lichtmond) in 1080p 3D OAU, but unfortunately only with 2 channel German audio. I was able to obtain an ac3 file that represents the English 5.1 audio track. Is there a way to “name” that audio file correctly to place it into a sub-folder with the mkv file so that Plex might recognize it then as an additional audio track (similar as we do with srt subtitle files), or do I have to remux the mov file and “swap out” the audio tracks (if so, what’s the best way to do that?)?
Many thanks!
Mike

You will have to remux new audio tracks.

Plex doesn’t support “sidecar” audio tracks.
You need to remux.
Take a look at MKVtoolnixGUI