I see your point, its not very useful to many people. Would something like this even be possible as a plugin? Can a plugin step in before MusicBee attempts to open the file with a video player and change the action? If it is I might attempt it myself.
If you don't mind changing the location of the video file you wanted to "play just the audio"; manually move said video file to another folder in your computer's music library. (— assuming you have a source music folder structure on your PC i.e. my "folder structure" consisted of 7 primary genre's (incidentally with over 34'000 thousand media files (subsequently scanned to MusicBee library)). After a year or so, I wondered why a video file ('MP4', but any video file has the same effect) stopped playing video externally (I use VLC player.),—by only playing audio, internally,—it was because I had unintentionally moved said video file to another location in my previously mentioned source folder on my PC. Having put the video file back in it's original location (manually on my PC) MusicBee played the video again perfectly.
...So, my abovementioned problem; video files playing audio internally not externally, might your solution!,—simply move the video file manually (on your PC) to another folder location in a previously scanned/mature MB library! Play the file in MusicBee and it plays internally.
NB: Please be mindful that moving files in this way is not recommended, I assume this, and I've seen this advised on this forum; the risk appears to be a potential to lose metadata/tags of the original file; so, backup (or copy) said file before you move it, as you might lose important tag data. I was fortunate, I didn't.
(Alternatively, if you simply change the video file name, in the source folder on your PC, by one or more characters', the same effect will work when you then play the file from MusicBee library; meaning you do not need to move the source file, as above, as a trade-off for manually changing the file name at source.).