iOS speak SDK

official referenct:https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/ApplicationKit/Classes/NSSpeechSynthesizer_Class/Reference/Reference.html

 

VSSpeechSynthesizer is available for the iOS - but it is a private API and as such will likely be rejected from the app store. However you can still make apps for private consumption. I consider this a bug as it makes creating accessible apps for the partially sighted for instance, harder. I have filed Bug ID #: 9451650 Bug Title: VSSpeechSynthesizer is Private

I currently use Flite TTS in my App. All hopes fix on the Nuance integration in iOS5 being available as a public API.

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Third party speaking SDK

 

Have a look a CMU's Flite TTS engine.

There are a few iOS ports of Flite, for example

 

I've heard that OpenEars is good, but I don't really know too much about it.