On the bottom slide up dock you have the option to select either the mute (bell shaped icon) or the lock/unlock icon option on the slide up dock or assigned to the slide switch on the side of your iPad.
To determine which one you would like assigned to either location go to settings
then General
You will see the option to set the physical switch on the side if your iPad to rather mute or lock. One selection will work from the bottom slide up dock and the other will work from the little button on the side of your iPad