Duplex audio is provided for a mobile communication device and an accessory. In some embodiments, the accessory can selectably operate in a duplex audio mode, concurrently sending audio to and receiving audio from the mobile communication device, or in another audio mode. In duplex audio mode, the accessory can enable its internal audio processing operations (e.g., echo cancellation) while the mobile communication device disables its corresponding internal operations or vice versa. The mobile communication device can control when the accessory transitions into and/or out of duplex audio mode.