“And all the people saw the sounds ...” (Ex. 20:15).
The Midrash calls our attention to an amazing aspect of the revelation at Sinai: the Jewish people were able to see what is normally only heard. What does this mean?
At their source, sound and sight are united. Only in our limited, physical world, in this alma deperuda (disjointed world), are these phenomena disconnected and detached. It is similar to our perception of lightning and thunder, which become increasingly separated from one another as the observer is more distanced from the source.
If we are bound and limited to the present, if we can only perceive the universe through the viewpoint of the temporal and the material, then we will always be aware of the divide between sight and sound. The prophetic vision at Mount Sinai, however, granted the people a unique perspective, as if they were standing near the source of Creation. From that vantage point, they were able to witness the underlying unity of the universe. They were able to see sounds and hear sights. God’s revelation at Sinai was registered by all their senses simultaneously, as a single, undivided perception.
(Gold from the Land of Israel p. 135. Adapted from Mo'adei HaRe’iyah, p. 491)