What sound grew louder and louder at Mount Sinai as Moses spoke with God?

Answer: A ram's horn (trumpet)

On the third day a very loud ram's horn (KJV: trumpet) sounded, and it grew louder and louder as Moses spoke.

“On the third day, when morning came, there was thunder and lightning. A thick cloud was upon the mountain, and a very loud blast of the ram’s horn went out, so that all the people in the camp trembled. And as the sound of the ram’s horn grew louder and louder, Moses spoke and God answered him in the thunder.”— Exodus 19:16, 19:19 (BSB)