We could say that this “loud voice” is needed in order to reach every ear around the globe. More fitting is to understand the loudness (literally, “great”) of the angel’s voice as expressing the indisputable authority and majesty of his Sender. The world’s inhabitants must hear, and listen.
7 And he said with a loud voice, “Fear God and give him glory, because the hour of his judgment has come, and worship him who made heaven and earth, the sea and the springs of water.”