Even when it used to Mute, it does not silence all sounds. Mute's function is to silence alarms, notifications, and other system sounds so that your iPad does not make embarrassing noises at the wrong moment; like in the middle of a meeting, movie, etc.
Sound that you intentionally create, like music or games, are normally not affected. Though depending on how the developer coded the app they may be.
To completely silence your iPad's speaker hold the volume down button for a couple seconds. It will quickly drop the volume to zero.