Okay here is the clue which the Heller is talking about- Brett Partridge was working for Red John, He came Jane to save him and recited that poem in his voice. In season 6 premiere red john called Partridge in that house and asked him to record a message for him in recorder. Then he killed him to confuse Jane. And when red john called Jane on his mobile he played that recording with the voice of Partridge as you can see he was painting Lisbon with his both hands (assuming he did't use loudspeaker on his phone). So from now on we will never hear that girlish voice from red john again.