First, double-check that audio is not muted via the in-game Settings Menu.
Then, check that your device is not in silent mode.
- iPhones have a Ring/Silent switch on the side of the device that will mute the game's audio. See https://support.apple.com/en-mide/HT203017 for details.
- On iPad, this is done via the Control Center using the button with a "bell" icon. See https://support.apple.com/en-us/HT210974 for details.