“in Horeb in the day of the assembly.” Here in Deuteronomy 18:16, Mount Sinai is called “Horeb,” and the “day of the assembly” was the day when Moses brought all Israel to the base of Mount Sinai to hear the words of Yahweh, and He spoke the Ten Commandments in a loud voice from the top of Mount Sinai. The record is in Exodus 19:16-20:21.
“Let me not hear again the voice of Yahweh my God.” This refers to God Himself speaking the Ten Commandments in a loud voice to the Israelites from Mount Sinai, which occurred between Moses’ third and fourth time up Mount Sinai. It was later, on Moses’ fifth trip up Mount Sinai, that God wrote the Ten Commandments on stone and gave them to him.
[For more on God speaking the Ten Commandments directly to the Israelites, see commentary on Exodus 19:9.]