If you've updated to iOS 11, swipe up from the bottom of your device's screen to change the volume on your device. If the volume slider is all the way down, this means your device is muted. To hear sound in your Nick Jr. App, move the slider up. Also, while watching a video, you can now tap the screen and use the slider that appears to change the sound level.

If that doesn't help, your device may still be muted. If it has a side switch, flip it to the off position so that you do NOT see the switch’s orange line. Then make sure the volume is turned up.