Dave, you don't need packman:gstreamer-plugins-libav to play MPEG-1/2 Video; you need packman:gstreamer-plugins-ugly-orig-addon. But once packman:gstreamer-plugins-libav get's installed then MPEG-1/2 Video playback stops working with the error. And then removing packman:gstreamer-plugins-libav MPEG-1/2 Video playback will work again (but so many other formats need packman:gstreamer-plugins-libav to play, so they'll stop working.)