I know this looks a pretty weird title, but a friend of mine had a similar trouble after installing Amarok 2.2 on Ubuntu 9.10 from the repos. Actually, in order to make it work, the package libxine1-ffmpeg is required, which probably isn't included in the default installation due to licensing issues. So, all you need to do is install the package and you are ready to amarok 
.
Update: As stated in the comment below by Alex, since Amarok is a Kubuntu application, installing kubuntu-restricted extras should do the job. Either click this link or run this command.
[shredder12]$sudo apt-get install kubuntu-restricted-extras
Although in non-ubuntu based distros you will have to go with the installation of libxine1.
Libxine1-ffmpeg contains MPEG-related plugins used by libxine1, the media player library used by Xine engine, which Amarok and other xine-based players use. This package includes the ffmpeg plugin for Xine, enabling xine-based players a large variety of audio and video codecs.
I didn't try this but after installation Amarok probably would have played files based on open audio formats (.ogg). This library was needed to enable it with appropriate codecs to play restricted formats.
Install libxine1-ffmpeg plugin on Ubuntu
Either click this link (in case of failure, help)or run the following command in a terminal to install libxine1-ffmpeg and get amarok in action.
[shredder12]$ sudo apt-get install libxine1-ffmpeg
Now, restarting amarok should fix the problem. I hope this will help most of the users who face a similar scenario of "Amarok won't play music/mp3 after installation on Ubuntu".
