Thunder is a sound which one hears, not sees. Yet, the nation was able to see the thunder: Ro’im es ha’nishma, “They saw what is (ordinarily only) heard.” This indicates that during the Revelation, the nation transcended human/normal physical limitations, rising to the level of superhuman comprehension, whereby they could see what had otherwise only been heard. Horav Tzvi Hirsch Ferber, zl, writes (Kerem HaTzvi) that he came across an innovative satirical explanation of ro’im es ha’nishma. At that time (early 20th century England/Europe) Jewish observance was hemorrhaging, decreasing with each passing day. One of the obvious reasons for this sad development was inconsistency. The children experienced a marked discrepancy between what they were hearing in school and shul and what they were observing at home. They went to school, where they heard all about the importance of Jewish commitment and Jewish observance. They heard it in shul, and it was drummed into them at their bnei mitzvah. At the end of the day, however, these were mere words which were superficial expositions, without any examples in real life. What they heard in school and what they saw at home were incongruous with one another, total contradiction. When children grow up hearing one thing but seeing another, they will usually adhere to what they see.
At Har Sinai, things were quite different. The nation was committed. What Klal Yisrael heard on Har Sinai was evident in the way they lived day by day. In other words: They saw what they heard.