A people voice is the voice that someone uses when talking to people who aren't their friends. This voice is automatically happy, nicer and sweeter than their normal voice. It is also often more high pitched. This is often the voice people use when answering a telephone or when working in retail. Similiar to the girlfriend voice.
She so used her people voice when she was talking to that customer over there. Did you hear how sweet she was? She never sounds like that normally!