If your Apple device is muted Nessy apps will not play sound, however you may still have sound on other apps.
There are a couple of ways to turn your device off 'Silent Mode' depending on the age of the device.
Step 1. Bring up the 'Control Centre' on your iPad or iPhone.
On a device with Face ID: Swipe down from the top-right edge. To close Control Center, swipe up from the bottom.
On a device with a Home button: Swipe up from the bottom. To close Control Center, swipe down or press the Home button.
Step 2. Tap the bell ensuring that it is not greyed out and therefore 'Silent mode' is off.
An alternative to this would be to make sure the switch on the side of your device is not activating 'Silent mode'.