Hearing, seeing, feeling, smelling and even tasting things that are not real, are known as hallucinations.
However, auditory hallucinations (hearing voices or other sounds that have no physical source) are the most common type in mental disorders.
Some doctors think that having hallucinations is characteristic only to schizophrenia, but, they are one possible characteristic specifically of Bipolar I Disorder, too.
Patients with manic symptoms who had some form of psychosis (loss of contact with reality) had auditory hallucinations half the time.

Do you take your medicines regularly (starting to experience hallucinations can be a sign that your disease is getting more severe)?
Are you worried about having thoughts of killing someone?
Do you find these thoughts unwanted that is coming as flashbacks?