All the people [at Sinai] saw the thunder and flames, the blast of the ram’s horn, and the mountain smoking. The people saw and shuddered, and stood at a distance.
Exodus 20:15
The above verse describes the scene at the Revelation at Sinai and what the Jewish people experienced as they received the Torah. It implies that the people saw the sound of the shofar, i.e., they saw what is normally only heard.
The difference between seeing and hearing is that once we have seen something, we are sure of its reality. If we only hear something, or hear about something, no matter how much the sound or explanation rings true, we still entertain a certain doubt about it.
G-d’s presence in this world is normally something we hear about but never see. At Mount Sinai, however, G-d’s presence became obvious. We saw it and perceived it as clearly as we normally perceive the physical world.
We can experience something of this consciousness whenever we study the Torah. Studying the Torah allows us to ascend to a level of awareness at which the reality of G-dliness is as evident as is the reality of the world.
—from Daily Wisdom 3, P. 140