We don't know what sort of voice the Self-Existent One used when speaking to Moses and Aaron, whether it was audible, or a strong, sub-vocal impression. But we do know that it was undeniable; it grabbed their attention.
If the phrase "Self-Existent One" is unfamiliar, it is the meaning of LORD, Jehovah, or Yahweh, because God told Moses, I Am That Which I Am, in response to Moses' question about His name(Exodus 3:13,14 YLT). He is the only being in the universe for whom the fact of His existence points to no one beyond Himself.
Anyway, the Self-Existent One clearly demonstrated through the series of miracles delineated in Exodus, chapters three through twelve, the work he planned to execute through Jesus. Establishing the theme of 2 Corinthians 5:17 Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come, He declared that this Passover would establish a new beginning for His people.
Some might ask why God had tens of thousands of innocent lambs slaughtered. To them it seems wrong to kill so many in the name of religion. Why would God demand that sort of bloody obedience?
God welcomes hard questions. For sincere inquirers, He always has an answer if they are willing to accept it. The believer in God's faithfulness would assume that He had a higher purpose in the lambs' slaughter, as in fact, He did. First, God demanded the paschal lamb be without blemish. That symbolism is obvious.
Second, while God's word doesn't distinguish between the paschal lamb's nature and its physical perfection, there is significance in the fact that God chose a sacrificial animal that is naturally prone to wander and get into trouble. The great I Am came in human form to demonstrate the wonderful truth that fallen, human nature does not necessarily hold mankind in an unbreakable strangle hold. Jesus become the Lamb of God, bleeding out and dying after the Romans drove spikes through His wrists and ankles, that we might enjoy the choice between bondage to our rebellious nature, or abandoning that accursed "old man." The question is, will we choose to remain sheep, mindlessly following our lower urges? Or will we choose to become the humans He created us to be?
It's as simple as that.