Want to animate a Monty Python-style talking head? Sure you do (and you can probably quote at least one third of Monty Python and the Holy Grail.)
Place your audio on one layer, and above it, place the head with its mouth and lower jaw erased.
On the top layer, place the mouth and lower jaw by itself.
Select the mouth/jaw layer and invoke Motion Sketch from the Window menu.
Click the Start Capture button in the Motion Sketch palette.
Gently start to drag the mouth/jaw layer in the Comp window. As soon as you start dragging, the audio will play back. As it plays, drag the mouth up and down in synch with the dialogue. AE will automatically add the keyframes.
When you're done, you can use the Smoother (Window>The Smoother) to reduce the number of keyframes for a tweaked animation.