Rashi explains they saw that which should be heard.
There is a famous saying that “seeing is believing”. It has never been suggested that hearing is believing. One may hear a lecture and be impressed so that he is emotionally moved. However, the response will not endure. He will go back to his way of thinking and original lifestyle. Judaism must be “seen” to be believed. It is stated in Tehilim: “Taste and see that Hashem is good.”
At Har Sinai the Jewish people responded with “we will do” before they said “we will listen”. It is necessary to perform, to live and to see it, and then a decision can be made regarding this way of life. If one were to stop and think and philosophize before he performed a mitzvah, he would seldom perform the mitzvah. One must see the beauty and experience the joy and satisfaction of “living Judaism” to understand it.
“And all the people saw the voices,” how did they see what should be heard? They saw the Ten Commandments in action. They perceived what a world would look like with Thou shall not kill. They saw what a family would look like with Thou shall not commit adultery. They saw what a home would look like with Honor thy father and mother, and with Observe the day of Shabbos. They saw Hashem in life. The voices were not just heard, the depth of the perception led to seeing the living activation of these voices. Seeing is believing!
