Deaf people do hear voices in their head -- even those who are prelingually Deaf.
Auditory hallucinations are the brain's way of compensating for a sense it does not have (hearing).
I've been hearing voices since 1991 (which I'm not ashamed to admit), but in my case, I grew up with hearing most of my life. The psychiatrist I saw at the time said that my voices were caused by the 4 different kinds of trauma I experienced during my childhood and teenage years.
However, after I had CI surgery and lost all of my residual hearing, I started hearing voices again (I heard up to 20 of them) which the attending psychiatrist said was due to sensory deprivation caused by the lack of hearing as well as a number of other factors.
I had to stay in the hospital for a little over a month at which time I was given high doses of mood stabilizers and antipsychotics.
I still hear voices to this day (I hear 6 voices -- 3 male; 3 female), but I was recently put on a different antipsychotic and no longer hear them except when I'm having a manic or depessive episode or have accidentally skipped a dose of my meds.