Unlike the iPhone, there's no dedicated mute switch on the iPad. There you are, in the back row of a funeral just minding your own business when, oops, you forgot to mute those YouTube clips on your iPad! However, you can still cut the sound in a hurry if you need to pretend you're doing something serious instead of watching the latest YouTube viral.
Apple decided to make the volume buttons do double duty. Tap them repeatedly and the volume will go up or down. But if you press and hold the volume down button for about two seconds, then you get a quick mute.