Why can't I hear the audio for a video?

You may have Silent Mode enabled in Control Center, which is only adhered to by certain apps like TrialPad (which is why you may be able to hear the audio in other apps). To change the setting swipe down from the top right to show Control Center then make sure the Bell icon is not red with a line through it. If it is red with a line through it, tap on it to turn Silent Mode off.