Ramban on Shmot 19:20

And the Lord came down upon mount Sinay, on the top of the mountain: and the Lord called Moshe up to the top of the mount; and Moshe went up.

AND THE ETERNAL CAME DOWN UPON MOUNT SINAI. If you will succeed in having insight into this section [of the Torah], you will understand that His Great Name, [i.e., the Tetragrammaton], came down upon Mount Sinai, and that He abode thereon in fire and spoke to Moses. The communication to Moses in the entire section was by this Proper Name of the Eternal. However, the ascent [of Moses — as mentioned in Verse 3: And Moses went up unto G-d] — and his bringing forth [the people out of the camp — as mentioned in Verse 17] — were towards the place of the Glory, as I have explained. And He warned, lest they break through unto the Eternal to gaze, because even the nobles of the children of Israel did not see Him, and all Israel heard the Voice of G-d out of the midst of the fire. It is this which Scripture says, And ‘Elokim’ (G-d) spoke all these words, just as our Rabbis have said: “Elokim designates the Judge.” And they have also said [with reference to the first two commandments]: “We have heard them from Hagvurah (the Almighty Himself).” In Deuteronomy it is written: These words the Eternal spoke unto all your assembly. That is because Scripture explains there [in the same verse] that He spoke out of the midst of the fire. And this is the sense of the verse, The Eternal spoke with you face to face in the mount out of the midst of the fire. And this is why it is said, I am the Eternal thy G-d. Now do not find a difficulty in what the people said to Moses, For who is there of all living flesh, that hath heard the Voice of the living G-d speaking out of the midst of the fire? They did not say “that they heard G-d speaking out of the midst of the fire,” but they said the Voice of G-d, referring to what they perceived. This is why they said, Go thou near, and hear all that the Eternal our G-d may say. And so did Moses say to them, Did ever a people hear the Voice of G-d speaking out of the midst of the fire? The word “speaking” here is referring to “the Voice,” something like the verse, and he heard the Voice speaking unto him.
From this you will understand what the Rabbis have always said in Midrashic homilies, i.e., that the Torah was given in seven voices. They are the ones to which David alluded in the psalm: Ascribe unto the Eternal, O ye sons of might. This is also the number of times [that the Voice] is alluded to in this section of the Torah. The verse, And there were ‘koloth’ and lightnings, is written defectively and therefore counted as one; likewise, the verse, And all the people perceived ‘hakoloth,’ is missing the vav which signifies the plural, [and thus refers to only one voice]. Thus there are six [‘voices’ mentioned here in the section], and [in addition] it clearly says, And G-d spoke. In Deuteronomy, Scripture likewise mentions seven ‘voices’ in connection with the Giving of the Torah. In Tractate Berachoth, however, the Rabbis have said in the Gemara that the Torah was given in five ‘voices.’ That is because they counted only the voices which are concealed, while the two — i.e., I am the Eternal thy G-d and Thou shalt have no other gods before Me] — are explained in Scripture. The purport of this is that Moses our teacher was given the Torah in seven ‘voices,’ and he was the one who heard them and contemplated them. The Israelites, however, heard one voice, as it is said, a great voice, and it went on no more, and it is said again, Ye heard the voice of words, but ye saw no form; only a voice. Here also Scripture alluded thereto in saying, And all the people perceived ‘hakoloth,’ with one vav missing, [thus making it singular, ‘the voice’], for all voices appeared as one. By way of the Truth, [the mystic lore of the Cabala], this is Scripture’s intent in saying, G-d hath spoken once, twice we have heard this. The sections of the Torah are thus explained, without anything being changed with another.