Files don't play, apps play files. IOW, you have to have an app that will play that file type. If you don't, you get the unable to play message. Also, your player app may not be able to play through Bluetooth (not every app developer is a genius). Try
BTmono. Run the player first, then run BTMono. If it's on, turn it off first, then turn it on. (The developer is right, it IS a hack, but it's a good hack if your app has a problem playing through Bluetooth.)
(If you "play" the music file, you have no idea which app is playing it. If you're using the player and choosing music it sees, that's a different matter, and BTMono probably won;t help.)