To appear in Music Player dropdown in 'Default Applications' list, your app's .desktop file (the one in /usr/share/applications) should contain Categories=Audio;Music;Player; line. If it doesn't contain this line, try adding it manually and see if the app will appear in the Music Player dropdown.
If it has a .desktop launcher in '/usr/share/applications' and has '%F' after the exec command like this:
Comment=Edit text files
You should be able to choose it as a default application in the file manager, not sure about the default applications part, it should be there too but I'm not sure.
Edited: Had the ...