I think there are some break-in CDs available, but I've never used one. I typically just use an up-tempo rock cd.
You can greatly reduce the overall time of break-in if you can play the speakers around the clock for a few days at a time. For me, this typically means using "the ol' out-of-phase trick". Wire just one of the speakers backwards (+ to -, - to +) but leave the other speaker wired correctly. Now face the speakers directly at each other (tweeter to tweeter). Get them as close together as possible without touching. Now when you play music, any like signal sent to both of the speakers will cancel each other out and the overall volume you hear is greatly reduced.
A few extra hints: If you can play a mono signal to both speakers, the amount of cancellation will be much greater. You can cover the speaker pair with blankets, quilts or sleeping bags to further reduce the volume of sound in the room.
Before you do this, make sure you know at what volume position the sound is normally loud using the CD you are going to choose for the break-in. When you perform the break-in using out-o-phase, don't exceed the position on the volume knob that you knew was loud when the speakers were wired normally. This will prevent you from over driving the speakers.