Using Amarok1.4 on Kubuntu Feisty, I got a "No suitable demux plugin" bug message while trying to play my .mp3 files that were still working 2 or 3 weeks ago. After insisting on trying, a "No MP3 Support" window pops up, with an "Install MP Support" button. However, it freezes almost immediately. The files are working fine with other media players. I tried to uninstall and reinstall Amarok, but it didn't help. Any suggestions welcome. Thanks in advance. Ágoston

Have you read this?
http://amarok.kde.org/wiki/MP3

I checked it and I have all the relevant packages installed.