Opus should be supported (the latest version of Musicbee has BASS' opus library (bassopus.dll) among the other library files in its main program directory. WEBM is a container format usually for video and audio, no? You should strip out the audio stream (see HowTo below).
If you want to strip out the opus audio stream from WEBM's, you'll need ffmpeg (https://ffmpeg.zeranoe.com/builds/)
HowTo: https://superuser.com/questions/412890/lossless-extraction-of-streams-from-webm
ffmpeg -i "input.webm" -vn -acodec copy "output.opus"
That is unless Steven comes up with something else.
This
So this forces me having 2 different versions of the same file, doubling up my audio storage requirements.
The only thing I could do is re-encode the Opus file to another format, but I am sure that you are aware of the quality loss and artifacts that re-encoding introduces, which I obviously would like to avoid.
Correcting myself, I found a solution.
Because .ogg is supported by MusicBee, and ogg is a container, and ogg can also contain Opus audio streams (other than Vorbis), then with the help of ffmpeg it is possible to simply copy the Opus audio stream from the webm container to the ogg container, with no trascoding and consequently no audio quality loss. Ogg files can be read by almost any audio app today (including the DJing app I was talking about).
The command for doing that is:
ffmpeg -i "file.webm" -c:a copy "file.ogg"
It can also operate directly on opus files:
ffmpeg -i "file.opus" -c:a copy "file.ogg"
and of course in this case too there won't be any transcoding. It's just the opus audio stream being copied inside the ogg container.
Bonus: put this in a Windows batch file and it will do the operations above for all the webm and opus files in the folder where it is ran:
for %%a in ("*.webm") do ffmpeg -i "%%a" -c:a copy "%%~na.ogg"
for %%a in ("*.opus") do ffmpeg -i "%%a" -c:a copy "%%~na.ogg"
Anyway, support for .webm would still be a nice thing to have :)
P.S. I researched this whole topic in the last week myself starting from zero knowledge about codecs and containers so I'd appreciate a lot if someone could confirm this is all correct!! Thanks!