Set AVAudioSession category and options to mix with other audio and turn volume down/up as well as interrupt any spoken text.

duckspeak.swift

import UIKit
import AVFoundation

class ViewController: UIViewController, AVSpeechSynthesizerDelegate {
	let synth  = AVSpeechSynthesizer()
	let avsesh = AVAudioSession.sharedInstance()
	let voice  = AVSpeechSynthesisVoice(language: "en-GB")
	let avopts:AVAudioSessionCategoryOptions  = [
		.MixWithOthers,
		.DuckOthers,
		.InterruptSpokenAudioAndMixWithOthers
	]
	let avcat = AVAudioSessionCategoryPlayback

	override func viewDidLoad() {
		super.viewDidLoad()
		do {
			try avsesh.setCategory(avcat, withOptions: avopts)
		}
		catch let error as NSError {
			print("error setting audio category: \(error)")
		}
		self.synth.delegate = self
  }
	func speechSynthesizer(synthesizer: AVSpeechSynthesizer, didFinishSpeechUtterance utterance: AVSpeechUtterance) {
		do   { try avsesh.setActive(false) }
		catch{/*meh*/}
	}
	func speak(words:String) {
		let words   = AVSpeechUtterance(string: words)
		words.voice = self.voice // or pass this in, whatever
		synth.speakUtterance(words)
	}
}