Using
https://getmusicbee.com/forum/index.php?topic=11990.0 I have set up the subtitles, but the individual covers are still not available. Don't know if that can be done.
Another issue within boxsets is when one of the discs is a DVD, and I've seen some of the various posts re this is a music library. No issues with that.
This has probably been suggested before, but ..
In order to at least identify that a DVD or video file is part of the boxset I have set up a dummy FLAC file showing metadata to identify the video file.
For example, Paul Simon's Live In New York City has 2xCDs and 1xDVD. I have the CDs as disc 1 of 3 and 2 of 3, and the dummy FLAC as disc 3/3. The dummy FLAC shows the same album title as the CDs, but has "Live In New York City DVD" as the track title. So in the main panel showing album covers, I click on the album cover, I see three discs for this album. If I right click on the track in Disc3, and then search Windows Explorer I get to the actual MP4 file (or shortcut), and can play it.