The first Rashi on Parshat Vayikra explains that Hashem’s voice was extremely loud but Bnai Yisrael cold not hear it. Rav Moshe Feinstein in Darash Moshe asks that if Hashem’s voice could not be heard then for what purpose was it loud? The Gemara in Sota (2a) brings another example of a heavenly voice that is not heard by humans. The heavenly voice announces, “ The daughter of this individual is designated for that man.” Here too we can ask what is the purpose of this voice if it cannot be heard?
The purpose of the heavenly voice is so that one should feel as if he was commanded directly from Hashem, although he was actually taught the Mitzvot through Moshe. Since the voice that reached Moshe carried through out the world it was really directed to us. We should have been able to hear it were it not for the fact that we are unworthy. As such, the divine laws would be more stringent if only Moshe was directed to tell us.
This is the reason that at mount Sinai all Jewish souls where present, even those of future generations. The soul before it enters the body is not obligated to do Mitzvot. When the soul and body are joined together the knowledge that it previously heard the Mitzvot obligates the person to observe the Mitzvot.
The same is true with the heavenly voice that states, “The daughter of this individual is designated for that man.” This is a special Mitzva for him. Each person has a special Mitzva, to marry a particular woman. The heavenly voice constitutes an additional Mitzva. Our awareness of this Mitzva is from our sages, whose words constitute reality for us.