This sounds more like you're using totem, the default media player for Ubuntu. In that case just enable the universe/multiverse repositories and install gstreamer0.10-ffmpeg and gstreamer0.10-plugins-ugly
This sounds more like you're using totem, the default media player for Ubuntu. In that case just enable the universe/multiverse repositories and install gstreamer0. 10-ffmpeg and gstreamer0. 10-plugins- ugly