Power reverbs. The more power, the more echo. So when a magical person such as a witch, wizard or some other type of spellcaster is reciting the lines of a curse/spell, or when a seemingly omnipotent being makes itself known, their voices suddenly becoming booming and echo-y is practically a given. Same thing goes for when fighters, even the most mortal, are announcing their attacks. The phenomenon could be attributed to the character's supernatural qualities, but let's face it, echoes just make recitations cooler. See also: Power Glows, Power Floats, and Power Makes Your Voice Deep.