Let us learn a posuk with Rashi

The Torah indicates: "All of the people could see the sounds and the flames" (20:15).

Seeing "flames" makes sense, but the phrase "see the sounds" gives us pause. 
Rashi supplies two alternatives for understanding this strange combination of terms: 

1) Rashi changes our reading of the verse by inserting a pause: "all of the people could see [pause]" with the implication that "there was not a single blind person among them." The verse then reads as follows: "all the people could see, and (they heard) the sounds...". A possible solution.

2) Rashi: "They saw the voices, something that is impossible to hear under other circumstances...".

Rashi explains that the Torah alludes to the miraculous nature communications from Heaven: voices, but not of the human sort; sounds, but not perceived in the manner of "regular" sounds. 

On this view, the peculiar "sense descriptions" are the Torah's way of teaching that the words used to describe the giving of the Torah are taken from our ordinary experience but cannot do full justice to the event being described. This explanation is also reasonable. It makes sense that words torn from ordinary use and employed to describe the unique experience of revelation will always seem somewhat strange to us.

However, there is a third possibility embedded in the commentary that Rashi wrote and his explanations to other verses. 

Consider the verse "...See now that I am He, and no other God is with me…" (Devarim, 32: 39).

Rashi explains this command to "see" as "...understand from the punishment that I brought upon you…". In other words, the Hebrew re-u means "to understand".

Similarly, in Sefer Yehezkeel, "….Perhaps they will see [ulai yiru], though they are a rebellious house..." (12: 3). Rashi explains: "Perhaps they will 'understand'". Again, the Hebrew verb which means to "see" is said to mean "understand." 

With this in mind, Rashi's interpretations for our verse assume a more significant tone. Rashi could have simply said that "seeing the voices" means "understanding" them. It would imply that they understood the Torah that they had been taught. 

Rashi declined that option because he felt a more significant lesson was to be learned if we avoided a more literal (pshat) explanation in this instance. Although it included some degree of comprehension, the experience of receiving the Torah could not be 
described simply as "understanding". For Rashi, precisely the indescribable qualities of the experience were stressed in the verse.