Hearing voices is one of the many symptoms of schizophrenia, a mental illness explained in WebMD& slideshow. Brain scans may eventually help scientists explain the causes, symptoms, and treatments for the disease.
(This fits the description of my ex's uncle, brother, and his self. the Walker family tree has a long line of schizophrenic diagnoses and symptoms which are hereditary) Schizophrenia Symptoms - Understanding Schizophrenia - Mental Health
What is Schizophrenia? Definition, Types and Symptoms of Schizophrenia