The joints around the mouth are the bit of this joint-based rig that gave me the most trouble. The mouth is an awfully complex series of... things. Fortunately, I am not the first person to make a bone-based face! Once again, with the kindness of professionals in the field, I have navigated these treacherous waters.
So how to do it? Place a ring of bones around the lips, just inside the skin and slightly deep into the lip itself. If you put the joint on the edge of the lip, you wouldn't be able to rotate it and get a nice lip purse. The number of bones you place in a ring is dependent on how detailed you want to get. Will this character have closeups? You'll need a bunch. If not, three on top and three on the bottom would work.
It will be helpful if you can add one more joint after each of these, to help orient the lip joints. These would ideally be right on the edge of the lip, and otherwise ignored. I could not do this on my setup, because I am right up against the max joint limit per characters, but it would have saved me some time in orienting these buggers.
As for the tongue, a simple chain of joints will do the trick. And that concludes the face tutorial. You can do other bits like the nose and ears, but the basics are all there. Once these bones are nicely skinned to the face, you can build a rig on them and move them as you see fit. I chose to make a series of Set Driven Keys to move the lips into common poses for lip-synching.
If you would like to see an example of what I mean, here is the same file that I linked in part 1, only this time it has all the rig controls built on top of the basic skeleton. This is fairly close to what I'm actually using in my animated short. You are free to take this and mess around with it, but please don't sell my stuff.
The above maya project file is licensed under a Creative Commons nonCommercial Liscence.
Click on the link for details.