1. A computer-implemented method comprising: 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.