Use Adobe After Effects to lip-synch a character designed in Adobe Illustrator
The most important thing for this tutorial is to design a character in Adobe Illustrator with a perfectly straight line. As the CC Split 2 effect in Adobe After Effects only allows to place 2 control points one beside the other, it would not be possible to control a curved (smiling) mouth.
It is also important to import the Illustrator file within After Effects as a composition, so that the layer below can be displayed when the mouth is open.
The most difficult part is to link the expression value of CC Split 2's controllers to the voice over. To do so we right click the audio and select Convert Audio to Keyframes. The Amplitude object generated contains keyframes for Left and Right channels, as well as for Both. We don't need to use all of them. I normally pick the set of keyframes in Both and delete Right and Left.
The last part is to apply a modifier to the expression. It totally depends on you. My preference is to move the lower lip more than the upper lip. Hence I used /2 (divide by two) for the upper lip and *2 (multiply by two) for the lower lip.