I had exactly the same trouble. This happen when you try to play a movie before a mp3 audio file : with the plugin assistant you will install gstreamer0.10-ffmpeg which will allow you to read mp3 file but not to seek. instead of this, you need to install gstreamer0.10-plugins-ugly and you will be able to seek in the mp3 audio file.
I had exactly the same trouble. This happen when you try to play a movie before a mp3 audio file : with the plugin assistant you will install gstreamer0. 10-ffmpeg which will allow you to read mp3 file but not to seek. instead of this, you need to install gstreamer0. 10-plugins- ugly and you will be able to seek in the mp3 audio file.