18. The computer-readable storage medium of claim 15 , wherein identifying the speech synthesis context comprises: receiving a request to synthesize speech.
15. A non-transitory computer-readable storage medium storing instructions which cause a processor to perform operations comprising: identifying, via a processor, 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, in a storage device, 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.