Thanks; that solved it! For others who wish to add *.webm audio file support to MusicBee at this time:
1. Download and install basswebm.dll from the link in the first post into the main MusicBee folder;
2. With MB closed, open the Configuration.xml file (in main MB folder) in e.g. Notepad, and add the following line under the section <SupportedFiles> :
<FileExtension Value=".webm" TagFormats="0" Codec="0" />
3. MusicBee will now find, open, and play *.webm files with supported audio.
Note that while current basswebm.dll (6/21/2019) is marked as a "First Release" version, it has yet to appear in un4seen's "permanent" plugins list so your mileage may vary. Works fine here now with WebM-wrapped Opus audio. The same procedure should work for adding *.mka file support, though not tested here.