Exemplo de AVSpeechSynthesizer.

AVSpeechSynthesizer.m

// supondo que o device está em pt já

AVSpeechSynthesizer *synthesizer = [[AVSpeechSynthesizer alloc] init];
AVSpeechUtterance *utterance = [AVSpeechUtterance speechUtteranceWithString:@"O NSNull é o melhor evento sobre"];
utterance.rate = AVSpeechUtteranceMaximumSpeechRate / 4.0f;
[synthesizer speakUtterance:utterance];
    
utterance = [AVSpeechUtterance speechUtteranceWithString:@"i O S"];
utterance.voice = [AVSpeechSynthesisVoice voiceWithLanguage:@"en-US"];
utterance.rate = AVSpeechUtteranceMaximumSpeechRate / 4.0f;
[synthesizer speakUtterance:utterance];
    
utterance = [AVSpeechUtterance speechUtteranceWithString:@"do Brasil!"];
utterance.rate = AVSpeechUtteranceMaximumSpeechRate / 4.0f;
[synthesizer speakUtterance:utterance];