From Amarok Wiki
Since Kubuntu 6.10 (Gutsy Gibbon) the Amarok package ships with a special feature which allows to easily install MP3 support on any Kubuntu version released after the year 2006.
This usually works perfectly, however it's possible that the installation fails. In this case just download the package for libxine1-ffmpeg and install it manually.
Unfortunately this only installs playback support, thefore you might want to install kubuntu-restricted-extras using your prefered package management application (e.g. Adept). kubuntu-restricted-extras will install MP3 support for the MusicBrainz tagging feature and install the k3b MP3 plugin. Note that it will probably make sense to install this package on Ubuntu/Xubuntu as well, since it only depends on KDE applications which are used by Amarok anyway.