If you do not hear sound from the Starfall app, the first step is to make sure that the device has sound enabled and that the volume is set properly.
If your device has a Ring/Silent switch, move the switch forward — toward the device's display — so that orange is not showing. (See How to Get Sound - iPhone for a quick visual guide.)
If that didn't help or your device doesn't have a Ring/Silent switch, open Control Center and find the icon that looks like a bell. If the bell icon's tile background is white, tap it to disable Silent mode. For help opening and closing Control Center, please refer to these Apple articles:
iPhone and iPod Touch: https://support.apple.com/en-us/HT202769
If these solutions do not work for you, please check the Apple support page for more information.