Accessibility.speak API not working on iOS, but works on Android

The Accessibility.speak function is working fine on Android, but not on iOS. I’ve tested on a real device iPhone 7 and in the simulator with iPhone X and iPad Pro 4th Generation.

import { Plugins } from '@capacitor/core';
const { Accessibility } = Plugins;

Accessibility.speak({ value: 'App is loading' });

I’m getting this in the console, but no audio plays…

callbackId: "123832822"
methodName: "speak"
options: {value: "App is loading"}
pluginId: "Accessibility"
type: "message"

Running the latest Capacitor

@capacitor/cli 2.2.1
@capacitor/core 2.2.1
@capacitor/ios 2.2.1
@capacitor/android 2.2.1

Any help appreciated, thanks.