There is a text that says that Moses heard the Divine voice from every direction: up, down, right, left.
It is even said that the Israelites heard with their toes.
In other words, at Sinai the Israelites became the amplifiers of the Infinite.
While remaining human, they ceased to be separate.
They heard the voice of God not from the outside but from the inside, from within their own beings.
This is the core meaning of the phrase: "And all the people saw the voices…"
When I perceive a sensation that comes from the outside, I hear it or I see it.
But here, the Divine penetrates man's being.
To say that God "descended" on Mount Sinai is to state that the Divine engulfed the material.
–Rabbi Adin Steinsaltz
From The Seven Lights, p. 241, by Rabbi Adin Steinsaltz