Hi,
posting this information for anyone who has the same problem. Amarok recently (I think it was when I upgraded to 1.4.6) stopped playing mp3s on my computer and instead displayed the yellow error box in the lower left corner. Musepack files still worked in Amarok, and mp3s still worked with other players. I installed the Debian package "libxine1-ffmpeg" , and apparently this solved it (for some reason, libxine1 only recommends that package - it doesn't depend on it).

Hello, i have the same problem but i try to do that and nothing.
I found a curious solution but i don't like it, when i run amarok int the console it say that "/bin/sh: /usr/lib/amarok/install-mp3: Permission denied", after this message i do "sudo chmod -R  777 /usr/lib/amarok/install-mp3/" and nothing only i can play mp3 file is running "sudo amarok" but how i said before i don't like this solution, please anyone know what can i do? please i love amarok but i can't use it because of that and i miss it.
Thanks.