15. A non-transitory computer-readable storage device encoded with computer program instructions that, when executed by 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, 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, to the client device, the audio data comprising the synthesized utterance of the selected text segment.
15. A non-transitory computer-readable storage device encoded with computer program instructions that, when executed by 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, 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, to the client device, the audio data comprising the synthesized utterance of the selected text segment. 17. The non-transitory computer-readable storage device of claim 15 , 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.