The covenant that Elohim made with the ancient Israelites at Mt. Sinai—to give them His special blessings and protection—was conditional on their obedience to His instruction. Elohim promised them, "Now therefore, if you will indeed obey My voice and keep My covenant, then you shall be a special treasure to Me above all people" (Exodus 19:5).
After hearing Elohim speak the Ten Commandments with His own voice, and witnessing "the thunder and lightning, the sound of the trumpet, and the mountain smoking," the Israelites pleaded with Moses, "You speak to us, and we will listen; but do not let Elohim speak to us, or we will die" (Exodus 20:18-19, NRSV).
Elohim accepted their request because the agreement He required from them was that they would obey His revealed instructions whenever and however they would receive them—whether from His mouth or from the mouth of His prophets. From that time forward His prophets acted as the "voice of the Lord" to the people. Notice how plainly this is confirmed 40 years later—long after Israel had received five full books of detailed instructions from Elohim through Moses.
At that time the Israelites had just reconfirmed their covenant to obey Elohim. So Moses assured them: "For the Lord will again rejoice over you for good as He rejoiced over your fathers, if you obey the voice of the Lord your Elohim, to keep His commandments and His statutes which are written in this Book of the Law, and if you turn to the Lord your Elohim with all your heart and with all your soul. For this commandment which I [Moses] command you today is not too mysterious for you, nor is it far off" (Deuteronomy 30:9-11). Moses' words clearly still represented Elohim's voice.
In other words, the content of the Sinai Covenant was not limited just to the instructions given at Mt. Sinai. The Israelites' agreement to obey the voice of the Lord included obeying instructions given through Moses long after they left Mt. Sinai.
Therefore, those who claim that the Sinai Covenant included only the Ten Commandments and possibly the extra information given in Exodus 20 to 24 fail to understand that the command to obey the "voice of the Lord" was far more expansive. It meant that the Israelites were to do whatever Elohim told them to do— with no limitations on when His instructions would be given. The only options were to obey or to refuse to obey Elohim's instructions, even when delivered through His prophets.