It’s not Pi Music Box or Mopidy or Music Box that aren’t supporting WMA or M4A files, it’s gstreamer that is having the issue. I found a post here from October 2017 (last post) that says “i had the same problem with wma files with gstreamer1.0 on arm, while having gstreamer0.10-plugins-good/ugly/bad/base installed I had to install gstreamer1.0-libav also and now it is working.”
It might not work out of the box, but I think it can be made to work.
[added] See also