My understanding is that if you hear voices only when you're manic or depressed, you have bipolar disorder with psychotic features. If you hear voices outside of a "mood episode" the (controversial) diagnosis is schizoaffective.
Medication may help but for some people, it doesn't or it doesn't help them entirely. Here's some resources that may be helpful to those who hear voices.
-
Intervoice: The International Community for Hearing Voices
-
Redefining Hearing Voices
-
Hearing Voices (Auditory Hallucinations)