This interesting little tidbit from a Finland-MIT collaboration - by functional brain imaging, scientists were able to show that auditory cortex (the hearing part of the brain) becomes activated when a subjects watches someone talking. So we really 'hear' what a person is saying when we watch their lips. A good tip for children with auditory processing difficulties. Children don't always know to sit up front and look at a teacher's lip movements, but it may help a great deal with comprehension.
Reading Lips Activates Auditory Cortex