To me this seems an interesting new idea that does make some sense.
But, I do have some doubts how this would benefit a larger amount of MusicBee users.
How often, and in what circumstances would you run into an MKV file, and only want to listen to the audio of it?
This might be a likely candidate for a new plugin.
But I am not so sure about having it as an integrated feature in MusicBee by default?