1. A computer-implemented method comprising: detecting a context associated with a mobile computing device, the context indicating that the mobile computing device is traveling in a vehicle based at least in part on a calculated speed of the mobile computing device; determining, based on the context, to switch the mobile computing device from a first mode of operation, during which the mobile computing device does not monitor ambient sounds for voice input that indicates a request to perform an operation, to a second mode of operation during which the mobile computing device monitors ambient sounds for voice input that indicates a request to perform an operation; in response to determining to switch to the second mode of operation, activating one or more microphones and a speech analysis system associated with the mobile computing device; and providing output using the mobile computing device, the output being responsive to voice input that is detected in a stream of audio data and that indicates a request to perform an operation.