There are some 3rd party libraries and API which are used to convert TTS ( TEXT TO SPEECH ) in iphone
1) Sphinix ( Not Efficeint )
2) Open-Ear ( Do not Required Internet Connectivity offline TTS )
3) Google TTS (Requires Internet Connection)
4) Acapela (Not Free You Have to pay to get SDK)
5) Objectuve C Speech Synthesizer Speak Engine (Do Not Required Internet Connectivity One of the Good offline TTS )