To model a mouth in c4d start with selecting points on the model that would like the size to be then move them to the shape you would like. Polygon select the points and "extrude inner". Then select those polygons and "extrude" them to the back to make the throat. On the outside extrude the polygons surrounded the extruded inner points to make the lips. Grab the polygons underneath the extruded inner points to make the teeth. Select the polygons on the back of the throat and extrude them to make the tongue. As you are extruding the polygons you can scale them however you please.