5. A non-transitory computer-readable memory including instructions that, when executed by a processor, cause performance of operations that comprise: receiving, by a mobile device, a voice input; generating, by the mobile device, two or more candidate transcriptions from the voice input; generating, by the mobile device, sensor data that measures an ambient condition surrounding the mobile device at a time when the voice input is received; inferring, by the mobile device, a user activity in which a user of the mobile device is engaged when the voice input is received, using the sensor data; selecting, by the mobile device, one of the two or more candidate transcriptions that corresponds to the user activity; and performing, by the mobile device, a command that is associated with the selected candidate transcription.
5. A non-transitory computer-readable memory including instructions that, when executed by a processor, cause performance of operations that comprise: receiving, by a mobile device, a voice input; generating, by the mobile device, two or more candidate transcriptions from the voice input; generating, by the mobile device, sensor data that measures an ambient condition surrounding the mobile device at a time when the voice input is received; inferring, by the mobile device, a user activity in which a user of the mobile device is engaged when the voice input is received, using the sensor data; selecting, by the mobile device, one of the two or more candidate transcriptions that corresponds to the user activity; and performing, by the mobile device, a command that is associated with the selected candidate transcription. 8. The non-transitory computer-readable memory of claim 5 , wherein generating sensor data comprises receiving, by the mobile device, sensor data that is collected by other devices that are within a predetermined distance of the mobile device, or that are in communication with the mobile device over a network connection.