1. In a computing environment, a system comprising, a service that includes a user interface accessible to clients via a network, a text-to-speech engine, and a data store of user-defined voice personas, a user-defined voice persona specifying one of a plurality of base voices and a plurality of voice morphing parameters associated with the base voice, the service configured to receive definitions of the voice personas from users and store the user-defined voice personas in the store of voice personas, where the users use the user interface to input new voice morphing parameters to modify the morphing parameters of the voice personas, the service configured to obtain via the network a user-provided text-to-speech input script comprised of portions of text comprised of respective voice persona identifiers, each voice persona identifier identifying one of the user-defined voice personas including a voice persona having the voice morphing parameters modified by the new voice morphing parameters inputted through the user interface, and the service converting the text-to-speech input script to a speech waveform via a text-to-speech engine based on the identified user-defined voice personas in the data store of voice personas, where portions of text in the text-to-speech script are converted to speech portions of the speech waveform using the user-defined voice personas identified by the voice persona identifiers, respectively.