Hearing mediums are those who hear the voices of spirits. This is a common type of mediumistic faculty.  As the spirit desiring to communicate approaches one such medium, and the two of them establish a fluidic connection (via their perispirits), there are two ways in which this medium could hear the spirit's "voice".  Sometimes, the perispirit of the medium receives the thought of the spirit, and the medium feels it as if it were an inner voice, speaking to his inner consciousness.  Other times, the connection between the communicating spirit and the medium occurs in such a way so as to make an impression on the auditory nerves of the medium. In this case, the medium hears the "voice" of the spirit as an external voice, just as if he were hearing the voice of another person.

A hearing medium may be able hear the voice of spirits in both ways or only one.  The medium can converse with the spirits, acting as an interpreter to pass on to other incarnates what the spirit wishes to say  ̶  either by repeating the message or writing it down. These mediums are very common. 

Kardec, Allan. "Mediums." The Mediums' Book. 2nd ed (1st edition FEB). Trans. Anna Blackwell (translated 1876). Ed. Livraria Espírita Allan Kardec. Brasilia-DF, Brazil:  Federação Espírita Brasileira [Brazilian Spiritist Federation}. 1986. 178-179 (item 165), 201 (item 190).

Rigonatti, Eliseu. "Mediuns Audientes" [Hearing Mediums] A Mediunidade Sem Lagrimas [Mediumship Without Tears]. Sao Paulo: Editora Pensamento Ltda. 2000. 17.