Notice in this picture and the next the shape of the statues' mouths. See how the mouth of this one is open and the one in the next picture is closed? Apparently, the open mouth is supposed to be emitting the sound "aaa" and the closed one "nnn," signifying the first and last sounds of the Sanskrit language. It's roughly analagous to "alpha and the omega" in Christianity.

(thanks to our resident iconography pro, Jarrod)