The sound isn't working
If the sound in the videos is working, but the sound of the app itself (background music, interface sound effects, etc) is not, then you likely have your device set to mute. Check the physical mute button (usually located above the volume) and make sure it's not set to mute.
Alternatively, if you can't hear any sound at all, from videos or otherwise, check to make sure your volume setting isn't turned all the way down.
If those solutions don't work, you can try this: