Wow, I just saw this simple little project a guy built, and I thought it would be pretty simple to make with a NXT, a sound sensor and a motor. Set the motor to position the mouth open or closed based on the intensity of the sound. Pretty neat.
If anyone does this, I would love to show it off, so let me know.
Update: I have already built one of these. It turned out pretty well. Am working on a video now and I expect to have it in a day or two.