Is hearing voices a sure sign of schizophrenia or some kind of psychotic disorder?

Not Sure, Just Maybe: One often associates auditory hallucinations with schizophrenia, but not all schizophrenics experience auditory hallucinations. They can also occur with mood disorders but usually to a lesser degree. Auditory hallucinations have also been known to manifest as a result of intense stress, sleep deprivation, drug use, and errors in development of proper psychological processes.

No: The neurologist oliver saks has a great new book out "hallucination" in which he describes the many sources of things like hearing voices. If you are not part of a cultural group where hearing voices is normal and are hearing voices it is a good idea, however, to see a psychiatrist to figure out what is going on.