Anâhata Shabda (Sk.). The mystic voices and sounds heard by the Yogi at the incipient stage of his meditation, The third of the four states of sound, otherwise called Madhyamâ—the fourth state being when it is perceptible by the physical sense of hearing. The sound in its previous stages is not heard except by those who have developed their internal, highest spiritual senses. The four stages are called respectively, Parâ, Pashyantî, Madhyamâ and Vaikharî.
Source: H.P.Blavatsky - The Theosophical Glossary