I had the same problem with mplayer plugin on firefox. i could see the video on www.quicktime.com/movietrailers
but could not hear the sound. Here is what i did to fix it.
extract all-20050115.tar.bz2 to /usr/lib/
must have root privileges to do that.
name the folder "all-20050115" to win32
so now you must have a folder /usr/lib/win32 with all the codecs necessary for mplayer.
now try watching the movie again and see if it works. If it doesn't let me know.