Not all videos have sound, but if you believe that the video you are watching should have sound and you're not hearing it, check these things:
1. Make sure your volume is turned up. Some devices have different volume levels for different applications.
2. Turn your ring/silent switch to "on" or turn vibrate "off." If you have your ringer turned off, many apps including ours will assume you do not want to be disturbed and mute the audio.
3. If you do not have a ring/silent switch, check your quick-access settings for a mute button. On Android devices this may require swiping from the top of the screen. On Apple devices this may be swiping up from the bottom.
If these steps do not work, send an email to email@example.com detailing the app you are using, your device's make and model, and the video or videos you are having trouble with.