Your iDevice has a physical mute button that silences all sound except the music/video player.
You can verify that your device is not physically muted by going into the Chess at ICC Menu and selecting Settings. In the General section, Look for Sound. Ensure that the sound is toggled on. If you see a gray speaker icon with an X beside the toggle, this would indicate sound in the app is turned on, but your device's physical mute button is also switched on. If the mute button on your device is switched on, this prevents our app from playing sounds, even when sound is enabled. The solution is to toggle your device's mute button to the off position.