Just wanted to mention that with iOS7 came a new speech synthesis API. Google doesn't seem to think anyone searching for "iOS sdk speech synthesis" would be very interested in looking at any Apple documentation.
Here's the link to the AVSpeechUtterance class reference:
https://developer.apple.com/library/ios/documentation/AVFoundation/Reference/AVSpeechUtterance_Ref/Reference/Reference.html
Here's the link to the AVSpeechUtterance class reference:
https://developer.apple.com/library/ios/documentation/AVFoundation/Reference/AVSpeechUtterance_Ref/Reference/Reference.html