13. A computer-implemented method for a network service allowing users to create and use voice personas in a text-to-speech system, the method comprising: maintaining a database of voice persona records, each voice persona record specifying an identifier of a voice persona, a base voice of the voice persona, and a plurality of voice morphing parameters of the voice persona; receiving from clients, via a network, specifications for voice persona records, the specifications comprising voice morphing parameters inputted by users, and in response modifying or creating voice persona records in the database that have the voice morphing parameters by modifying the voice persona records with the voice morphing parameters inputted by the users; receiving from clients, via the network, text-to-speech scripts, a text-to-speech script comprising portions of text and identifiers identifying voice personas that have the voice morphing parameters received from the clients, and in response: using the identifiers to retrieve corresponding voice persona records identified by the identifiers, for each retrieved voice persona record, given such a retrieved voice persona record, performing text-to-speech conversion on a corresponding portion of text in the text-to-speech script using the base voice specified by the given voice and morphing the base voice according to the voice morphing parameters specified by the given voice persona record, the conversions of the portions together producing an audio speech data unit comprised of portions of audio speech data of the text portions in voice according to the respective voice persona records.
13. A computer-implemented method for a network service allowing users to create and use voice personas in a text-to-speech system, the method comprising: maintaining a database of voice persona records, each voice persona record specifying an identifier of a voice persona, a base voice of the voice persona, and a plurality of voice morphing parameters of the voice persona; receiving from clients, via a network, specifications for voice persona records, the specifications comprising voice morphing parameters inputted by users, and in response modifying or creating voice persona records in the database that have the voice morphing parameters by modifying the voice persona records with the voice morphing parameters inputted by the users; receiving from clients, via the network, text-to-speech scripts, a text-to-speech script comprising portions of text and identifiers identifying voice personas that have the voice morphing parameters received from the clients, and in response: using the identifiers to retrieve corresponding voice persona records identified by the identifiers, for each retrieved voice persona record, given such a retrieved voice persona record, performing text-to-speech conversion on a corresponding portion of text in the text-to-speech script using the base voice specified by the given voice and morphing the base voice according to the voice morphing parameters specified by the given voice persona record, the conversions of the portions together producing an audio speech data unit comprised of portions of audio speech data of the text portions in voice according to the respective voice persona records. 17. A method according to claim 13 , wherein the speech-to-text conversion uses a hidden Markov model-based system, and wherein the morphing is performed using a sinusoidal model based morphing algorithm, a source-filter model based morphing algorithm, or a phonetic transition morphing algorithm.