I think people pray loudly so that they can be seen (heard) to be praying. They want everyone else to know they are praying. "Look, I am praying" they announce to the world, as they prey at the top of their voice. They want to be seen as "devoted." They are "showing off" - I can pray louder then you! They want everyone to know they are "followers." God is not deaf. God will hear the quietest prayer from the smallest child, unvoiced. Who responds well to be shouted at?