One thing I've noticed when you get a TV broadcast that has 5-channel sound, it that you get the voices on the center channel. Particularly with things like voice-overs, sportscasters etc. If you can mute the center channel, you can watch a game, for example, with no commentators.
But sometimes you do want to hear what they said, if there is something on the field you don't understand. If you have a PVR, you can rewind and turn on the center channel audio (though rarely is there a good UI to do this) but here's another idea.
If the center channel is muted, offer a button that shrinks the display and puts up the last minute or two of history of the closed captions. Of course if there are already bars on the screen use those to display this. You can read quickly (let one of the scrolling controls let you move around) and figure out what was going on, faster than you could through replaying.