On the iPhone: please check that your device is not in silent mode. If the ringer is set to vibrate the app will not make sound.
On the iPad: please make sure first that the switch on the right side of your iPad is not set to mute. Then please go to the Settings app, in the "Sounds" menu, and drag the slider to adjust the volume.
For both devices, please note that some audio features like the Read To Me button require an active internet connection.
If issues persist, contact us at firstname.lastname@example.org and we will help you.