how to synthesize audio in iPhone

Question

I need to synthesize a voice so that it can be used to speak the text that is supplied to it. how can it be done in iPhone. I went through Sphinx voice synthesizer and other voice synthesizers, but they have some predefined voice for converting text to speech. In my app I require the voice provided by me to respond back.

Any suggestions in this regarding where to start with, is very helpful.

Thanks in advance.