First. you should install the
libmad package from the
rpm-fusion repositories. That should fix most of your problems. (The version of
mplayer the
rpm-fusion repositories, IIRC, includes some propitiatory extensions which make it somewhat more versatile.)
Second, check the "application associations" settings for MP3 files, and change it as appropriate so some other, or no, app starts for such files.
Note that Fedora does
not provide support for mp[1234] files since those formats are patent-protected, and, therefore, not FOSS.