Alex brought up the point that if when thinking we "hear a voice" in our head - what is the experience for someone who is deaf from birth.
This episode of Radiolab is completely relevant and mentions the possibility in schizophrenia research that that type of hearing voices might have to do with the symptom of not recognizing your internal voice.
My feeling (without diong any investigation into the matter) is that we can't confine the internal voice to a passive auditory like modality (perhaps there is FMRI research on thinking and parts of the auditory cortex lighting up); there must be an active/motor component as well of forming the words (must check FMRI stuff again). So I wonder if the whole experience for someone deaf from birth is akin to whatever sensory/motor experience that that same someone feels when talking normally (eg. motor/tactile act of signing). I will have to check if there is any literature on this.
Final thought - I wonder if there is reduced incidence of "hearing voices" type psychosis for the deaf population compared to the rest of the population. Must check.