Try going here: http://www.apple.com/trailers/sony_p...ck/medium.html
Does that work? In my case, the video plays perfectly. If I right click on the video and select "configure", it says "X11" selected for the video driver, "oss" driver for audio, "512" as minimum cache size, and "25" as percent media to cache. All check boxes are selected except the last two (play media directly without caching and connect to RTSP media over TCP).
I am using Ubuntu 6.06 which lists mozilla-mplayer package as version 3.17-1ubuntu1. The version of mplayer is 0.99+1.0pre7try2.
I didn't compile anything myself, but installed everything from ubuntu repositories using apt-get.