There is also an older program called Elastic Reality that you could probably find on the net for free that does bezier line morfs and warps, and you use it to create a masked area of lips and mouth then animate those lines. Another way would be to create three layers, one for each top and bottom mouth, then the rest of the character without the mouth elements. The mouth layers on top, and use bezier warp effect that you can either animate by hand or use the audio expressions and import the v/o to animate the motion of the mouth parts.