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, and the user context indicating a level of complexity of speech that the user is likely able to comprehend at a given time when the context data was received; selecting, by the one or more computers and from among a plurality of candidate text segments that correspond to different levels of complexity of speech, the text segment for text-to-speech synthesis by a text-to-speech module that best matches the selected user context; generating, by the one or more computers, audio data comprising a synthesized utterance of the selected 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 selected text segment.
2. The method of claim 1 , further comprising: receiving, by the one or more computers, data indicating a voice query detected by the client device; and generating, by the one or more computers, the plurality of candidate text segments, each candidate text segment responsive to the voice query, wherein receiving the context data comprises receiving context data indicating a current context when the voice query was detected by the client device, and wherein providing the audio data comprises providing the audio data to the client device for output as a response to the voice query.