God 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 Exodus 19:5Now therefore, if you will obey my voice indeed, and keep my covenant, then you shall be a peculiar treasure to me above all people: for all the earth is mine:
American King James Version×).
After hearing God 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 God speak to us, or we will die" (Exodus 20:18-19 Exodus 20:18-19  And all the people saw the thunder, and the lightning, and the noise of the trumpet, and the mountain smoking: and when the people saw it, they removed, and stood afar off.
 And they said to Moses, Speak you with us, and we will hear: but let not God speak with us, lest we die.
American King James Version×, NRSV).
God 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 God through Moses.
At that time the Israelites had just reconfirmed their covenant to obey God. 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 God, to keep His commandments and His statutes which are written in this Book of the Law, and if you turn to the Lord your God 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 Deuteronomy 30:9-11  And the LORD your God will make you plenteous in every work of your hand, in the fruit of your body, and in the fruit of your cattle, and in the fruit of your land, for good: for the LORD will again rejoice over you for good, as he rejoiced over your fathers:
 If you shall listen to the voice of the LORD your God, to keep his commandments and his statutes which are written in this book of the law, and if you turn to the LORD your God with all your heart, and with all your soul.
 For this commandment which I command you this day, it is not hidden from you, neither is it far off.
American King James Version×). Moses' words clearly still represented God'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 God 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 God's instructions, even when delivered through His prophets.