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.
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. 5. The system of claim 1 , wherein: applying the rule or the probabilistic model to sensor data further comprises applying the rule or the probabilistic model to data that is sensed by a first sensor of a mobile device; commencing the background audio recording further comprises activating a second sensor of the mobile device in response to inferring that the user is likely to initiate voice input.