Snap Scene has no audio output on an iPad

This is usually the slide switch on the side of the iPad.
Check in your iPad's general settings for the "Use side switch to:"
Change it to Lock rotation and bring up the volume using the external volume up. It will show an icon as Sound effects volume.

If you don't have a switch on the side, swipe up on the screen to show the control center.
https://support.apple.com/en-us/HT203121 
or depending on the model of your iPad,  swipe down from the top-right corner of your screen
Make sure the Bell icon is not Red.