Welcome to the forum. Where is your core running, is it Rock or a Linux server? In this case you may have to install ffmpeg to provide the necessary codecs. On a Linux distribution, install them with the usual package manager. On ROCK, you have to do it manually:
If this is not it, then I don’t know what the issue might be, but there are other threads that may be helpful. In these threads, other people seem to be able to play m4a. Maybe you find something there, e.g.,