Please double check to make sure that your device isn't on mute mode?

On iPhones, there's a switch near the volume buttons that controls this, and while some app videos such as youtube will still work when mute mode is on, our videos don't. If you're on a tablet, you can check this by swiping up from the bottom of the screen.
You'll also want to ensure that your Media volume, in particular, is set high enough.
Lastly, if this doesn't prove successful, try going to 'Settings', tap 'Sounds', and under 'Ringer and alerts' toggle 'Change with Buttons' to off (not green).

You can find Apple's instructions for this process here: https://support.apple.com/en-us/HT208353


If this didn't help, or if you're using an Android device, please contact the elves HERE, and make sure to let them know what steps you've tried already.