I think there are people not on meds who hear voices and still question whether they're being truthful or not. At least from time to time they manage to doubt the voices.

According to Edward Podvoll in Recovering Sanity that's one of the techniques John Thomas Perceval used in pulling himself out of psychosis into reality. When he felt doubt, he'd strengthen the feeling. He had command voices which would threaten to harm him if he didn't do what they said. He would defy them, and then notice that they never carried out their threats.

Personally I think voices come from the mind of the person who is hearing them. But even if you believe they come from outside of you, you don't have to believe them.