Android
If the app is not speaking the words, please make sure that your device has got the Google Text-To-Speech engine installed (also known as Google TTS). If it is not installed, please install it from here https://play.google.com/store/apps/details?id=com.google.android.tts&hl=en_GB&gl=US
The app uses the default Google Voice on you device. You can change this in “Settings” -> “Accessibility” -> “Text-to-Speech output”
iOS
The app uses the “Spoken Content” voice on your device. This can be changed in “Settings” -> “Accessibility” -> “Spoken Content”