(15) All the people witnessed the thunder and lightning, the blare of the horn and the mountain smoking; and when the people saw it, they fell back and stood at a distance. (16) “You speak to us,” they said to Moses, “and we will obey; but let not God speak to us, lest we die.”

(1) (Ibid. 20:15) "And all the people saw the sounds and the lightnings": They saw what was visible and heard what was audible. These are the words of R. Yishmael. R. Akiva says: They saw what was audible and heard what was visual. There was nothing that left the mouth of the Omnipotent which was not inscribed on the tablets, as it is written (Psalms 29:7) "The voice of the Lord hews (with) flames of fire."

And all the nation saw the sounds: I have previously explained the intent of seeing the sounds. All sensations join together and are processed at one point. And this is the meaning when they saw the sounds and the lighting. This drove a person to be afraid, and the voice of the shofar was something never heard before. And the mountain itself was smoke. When they saw these wonders, they trembled....

(1) רואים את, the word רואים as “seeing” is to be understood as like the same word in Kohelet 1,16 ולבי ראה, “and my heart ‘saw.’” Just as the heart cannot see, so people cannot “see” sounds. The meaning is that they understood the meaning of these sounds. They could not continue to endure the volume or nature of the sounds. They were afraid they would die.

(1) . וכל העם רואים את הקולות, “And all the people saw the sounds (thunder), etc.” The word “saw” in this context is to be understood as “understood, comprehended,” similar to when Yitzchak spoke of ראה ריח בני “look at the fragrance of my son” (Genesis 27,27). He was well aware that fragrance just as sound cannot be seen by the eye. He meant that just as the eye is used to understand something clearly, so he experienced a clear understanding of what his son’s fragrance represented. Similarly, here; the Torah describes that the aural perceptions of the people were as convincing to them as if they had experienced the same thing with their eyes, a normally superior tool for clear perceptions.

We have to understand how they could seen the sounds ... It sounds reasonable that each word the G-d spoke became palpable and tangible to the extent that it took on physical form and could be seen in the air as floating letters as is they were written in front of them....

.....the Seer looks at a thing in its completeness, exactly as it is. But for the Hearer, the sound changes as it enters his ears, and it isn’t exactly the same sound that was originally made. That’s the advantage of Seeing. But with Hearing, there is an advantage that the sound truly enters inside of him through the ear, whereas the sight remains outside. With this in mind, the verse teaches us that the Children of Israel had both advantages. They received the words in the manner of 'seeing sounds,' such that even though they truly entered inside of them, nevertheless they 'saw' the sounds, without any distortion