I hope this fulfills your request of a closer look at Benders mouth.
About his mouth and the electric motor behind that animates his speach.
Behind the backplate a system containing of rolls and a small electric motor is hidden,
the sides of his mouth is open and a transparent "belt" can be driven by the motor to give the impression similar to the "electric peaks" Bender shows when he "speaks" in futurama show. The belt goes between the vertical bars and the backplate with horizontal flatbars.
I hope you can see how his mouth deepens into Benders head,
metal bars make out the vertical side of the teeth giving the viewer a true 3D experience walking around him or if Bender turns his head.
The horizontal sides of his teeth are chromed flat bars raised from the surface.
The backplate is painted with white hammer paint.

I have not fitted a speaker and a speach generator simply because I suck and dont know how to program one without a L O T of reading up...

Anyhows, despite my crappy cellphone quality of the pic I hope I have answered your question about his mouth.
Thanks for your tremendous support mates, you are just the greatest motivators there is. :)