Even though we live in a time when such animal sacrifices are not possible, one of the main lessons which is pertinent to us is that all forgiveness for wrongdoing requires true "sacrifice" on the part of the perpetrator of the sin.
Moshe hears the voice of the Lord as God calls to him. However, it is not the same experience that it was at Mount Sinai. it is a private revelation exclusive to Moshe. The great heavenly voice is not heard outside the precincts of the Tabernacle itself.
What is the significance of this call? And why does it need to be made at all? After all, Moshe had already ascended the mountain of Sinai and been taught the Torah and its laws previous to this call.