1. A method performed by one or more computers, the method comprising: receiving, by the one or more computers, context data from a client device of a user; selecting, by the one or more computers, a user context corresponding to the context data from the client device, the user context being selected from among a plurality of user contexts; determining, by the one or more computers, a text segment for text-to-speech synthesis by a text-to-speech module based on the selected user context; generating, by the one or more computers, audio data comprising a synthesized utterance of the text segment using the text-to-speech module; and providing, by the one or more computers and to the client device, the audio data comprising the synthesized utterance of the text segment.