How can I put my Kindle Fire into a totally silent mode?

My Kindle Fire (original) plays a notification sound whenever someone playing Words with Friends with me makes a new move. As I write this, in the main settings, Volume is turned down all the way, extreme left. More, Sounds, Notification Sounds is set to "silent." Nevertheless, it plays a notification sound when a new move arrives. It even plays it if the Kindle is asleep.

Is there a way to mute the Kindle Fire fully, short of turning power off or plugging in earbuds?