Music is in all channels of a 5.1 mix. Dialog is really the only thing that you could cut out of 5.1, even then the reverb of the dialog as pertinent to the room will still be in the back channels.
Castro's voice may or may not be center channel exclusive. As it's from another source, who knows how they mixed it into the film. You could check, but if not just get the intro music from the CD.