“Do profoundly deaf people who learn to talk have a voice in their head?”
— Heather & Allen Exby
“My best answer to this,” Hauser wrote, “is that the brain has a special capacity to develop phonological representations, even when it does not have auditory input. The representations might be dramatically different from what hearing individuals hear. Nevertheless, they function in the mind as ‘sounds.'” Deaf schizophrenics, he continued, have auditory hallucinations, and blind schizophrenics have visual ones.
This is a good link. I think thoughts are just what they are, thoughts. They don’t really exist physically but from our minds and apparently, 99% of them express theirs phonologically while we express with our hands visually in Sign.