Prisoner Zero was a shapeshifter, able to change from one form to another by forming a psychic connection with another lifeform, and using an image from within their mind. Through this process, Prisoner Zero could even resemble multiple lifeforms, which included their voices.
Prisoner Zero could also duplicate the voices of those whose image it took. However, in the case of multiple lifeforms, he had trouble speaking with the right voice through the right mouth. (From 'The Eleventh Hour' 2010).