13. The system of claim 8 , wherein the local cache of text-to-speech units comprises speech snippets for use in concatenative synthesis.
8. A system comprising: a processor; and a computer-readable medium having instructions which, when executed by the processor, cause the processor to perform operations comprising: identifying a speech synthesis context; determining, based on a local cache of text-to-speech units for a text-to-speech voice and based on the speech synthesis context, additional text-to-speech units which are not in the local cache; requesting from a server the additional text-to-speech units; storing the additional text-to-speech units in the local cache; and synthesizing speech using the text-to-speech units and the additional text-to-speech units in the local cache.