Make sure that the mute button on the side of your device is off. If you see orange, that means that the device is muted and needs to be switched off. Some other iOS apps override the mute button, so you still may be hearing sound in other apps while the mute button is on.
On iPads, make sure that the side switch is set to control Mute and not Screen Lock:
- Go to your iPad's Settings.
- Scroll to "Use Side Switch to".
- Make sure that Mute is checked.