The first thing to check is that the volume is turned up in the app volume icon (top middle). The next thing will be to ensure you have ‘Speaking Clock’ ticked in the settings. If both of the above are true and you can still not hear the time spoken then please navigate to the Google Play Store and look for a TTS pack in your chosen language. The app uses the default Text to Speech engine built in to your device.
Try out SVOX on Google Play, they offer many voices in many different languages.