Thanks, this worked for me too.
Though, I had no /etc/mplayer/codecs.conf so I searched for it into the filesystem of my newly upgraded ubuntu gutsy box.
There was something like that in /usr/share/doc/mplayer/examples/etc/codecs.conf.gz so I gunzipped it, edited it by moving the "audiocodec mad" full section just before the "audiocodec mp3" section, and copied into /etc/mplayer .
So far, so good! Thanks, Nikem...