First, check that your volume is loud enough to hear the sounds. Careful, this is not the phone ringer volume, but the phone media playback volume that matters. On iOS devices, check that the mute button of your device (if it has one) is not enabled. If the problem persists, in the Verroo app, tap the Menu icon on the top left of the lock list, and choose Notification Settings. Check that the switch is turned on and tap the "Done" button on the top right of the screen.