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.