4. The system of claim 1 , wherein the sensor data comprises data that is sensed by a keypad sensor of the mobile device, data that is sensed by a position determination sensor of the mobile device, data that is sensed by an accelerometer of the mobile device, data that is sensed by a magnetometer of the mobile device, data that is sensed by a light sensor of the mobile device, data that is sensed by a proximity sensor of the mobile device, data that is sensed by a capacitive sensor of the mobile device, or data that is sensed by a touch sensor of the mobile device.
1. A system comprising: one or more computers; and a computer-readable medium coupled to the one or more computers having instructions stored thereon which, when executed by the one or more computers, cause the one or more computers to perform operations comprising: applying a rule or a probabilistic model to state data that references a state of a mobile device and sensor data that is sensed by one or more sensors of the mobile device; inferring, based on applying the rule or the probabilistic model to the state data and the sensor data, that a user of the mobile device is likely to initiate voice input; commencing a background audio recording in response to inferring that the user is likely to initiate voice input; and transmitting (i) an audio signal that encodes the background audio recording and an utterance, and (ii) duration data that indicates a duration of the background audio recording encoded in the audio signal.