If you’re using mplayer plugin in Ubuntu linux and it doesn’t work chances are you haven’t configured the /etc/mplayerplug-in.conf file. My mplayer-plugin was loading and buffering when playing embedded media files but refused to start playing and often crashed firefox. If I saved the media and then opened it with mplayer it played fine!
I did a quick google and found that mplayer plugin has it’s own config file for mplayer’s options under /etc/mplayerplug-in.conf . In Ubuntu all of the options in the file are commented out by default so you have to go in and configure what sound output engine you prefer (esd if you’re using the default Gnome install). This is what was crashing mplayer for me, once I set esd as the default output all my media played fine.