11. A system comprising: one or more computers; and a non-transitory computer-readable medium coupled to the one or more computers having instructions stored thereon, which, when executed by the one or more computers, cause the one or more computers to perform operations comprising: receiving context data from a client device of a user; selecting 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, 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 audio data comprising a synthesized utterance of the selected text segment using the text-to-speech module; and providing, to the client device, the audio data comprising the synthesized utterance of the selected text segment.
13. The system of claim 11 , wherein the operations further comprise generating each of the plurality of candidate text segments from one or more search results identified by a search engine in response to a query from the user.