This blog includes the use of -

AVSpeechSynthesisVoice

 If we want to change the default voice i.e system voice then we need to use speechVoices{} array which contains the name of persons and locations. iOS has a nice method for setting the voice but there is only support for a single identifier, the Alex voice.

Here is the Code-

let voice = AVSpeechSynthesisVoice(identifier: AVSpeechSynthesisVoiceIdentifierAlex)

To use another voice, the array of voices can be searched for a matching name. The identifier in the previous call does not match the name of a voice as you might imagine it to.

var voiceToUse: AVSpeechSynthesisVoice?
for voice in AVSpeechSynthesisVoice.speechVoices() {
    if #available(iOS 9.0, *) {

         // use the name that you want to use from array. e.g.- "Karen"
        if voice.name == "Karen" {
        voiceToUse = voice

        }
    } 
} 

To use voice, assign the voice using the voice property on an utterance and then speak the utterance.

let utterance = AVSpeechUtterance(string: "Hello from iOS.")
utterance.voice = voiceToUse
let synth = AVSpeechSynthesizer()
synth.speakUtterance(utterance)

Also we can set the pitch and rate of voice by using properties of AVSpeechUtterance.Find the Code here-

//set the rate of voice
utterance.rate = 0.5