I would to convert user voice to a string by using nsspeechsynthesizer or other ios7 way. I can only find a way to do the opposite.
The NSSpeechSynthesizer
class only exists on OS X.
iOS 7 introduced AVSpeechSynthesizer
and related classes. But as the name suggests, these are for synthesizing speech — text to speech — not recognizing it (speech to text).
iOS provides no system API for the latter, but there are several third party speech recognition systems available that you could search for.