Audible alerts are on by default. To hear them you must adjust your device's sound settings.
iOS:
Ensure your device is in ring mode using the switch on the side.
Ensure your volume is turned up, either using the switches on the side of your device, or go to Settings > Sounds. To set the volume for all sounds, drag the slider below Ringers and Alerts to a sufficient level.
Android
Open your device's Settings app.
Tap Sound.
Ensure that your device's Ring and notification volume is at a sufficient level to hear.