Ad audio is set by media volume. While in-game sounds may be disabled, ad-audio may play if the ad is tapped and your device volume is enabled, (not on Vibrate/Silent mode.)

 

To adjust your media volume, press the Volume down button on the side of your device. To completely disable sound, we recommend depressing the switch on the left side of your device to enable Vibrate/Silent mode.

 

Android Support Article: Change Volume Settings