A method for communicating messages by a mobile device via a sound medium is disclosed. The mobile device receives input sounds from at least one mobile device via the sound medium. From the input sounds, an input sound signal carrying a first message encoded with a first key is detected. The mobile device decodes the first message based on a matching key. An output sound signal carrying a second message encoded with a second key is generated. Further, the mobile device transmits an output sound corresponding to the output sound signal via the sound medium.