A USE flag for mplayer support would be nice. see also http://www.emacswiki.org/emacs/BongoMPlayer
Ebuild with MPlayer support (and IUSE="mplayer") committed to CVS. We may have to revise this later, since Bongo is scheduled for inclusion with Emacs, and at that point a separate app-emacs/bongo-mplayer package may be needed. However, it looks like Bongo won't make it for Emacs 23.1; so this will not be in the near future.