Ramban

IN A THICK CLOUD. This is the thick darkness where G-d was, and all the people saw it and recognized it as such, as it is said, And the appearance of the Glory of the Eternal was like devouring fire on the top of the mount in the eyes of the children of Israel., 24:17.
IN ORDER THAT THE PEOPLE MAY HEAR WHEN I SPEAK WITH THEE. Rabbi Abraham ibn Ezra commented that there were among the Israelites people who doubted the existence of prophecy. And even though it is written, and they believed in the Eternal, and in His servant Moses, it is said there, and ‘Israel’ saw, but not “all Israel.” It is this which they said to him [after the Giving of the Torah], This day we have seen that G-d doth speak with man, and he liveth, for at first they did not believe so. This is the meaning of the verse here, in order that the people may hear when I speak with thee, the Ten Commandments, and also believe in thee forever, i.e., “that you are My prophet,” for henceforth the matter of prophecy will be confirmed to them. [Thus far the words of Ibn Ezra.] This is not correct. The children of Abraham never doubted prophecy, as they had always believed in it since the time of their forefathers. Scripture already has stated so: And the people believed, and they heard that the Eternal hath remembered the children of Israel; and they believed in the Eternal, and in His servant Moses. Even though it does not say there, “and all the people [believed]” or “all Israel believed,” neither does it say here “that ‘all the people’ may hear.” The correct interpretation appears to me to be that G-d said to Moses: “I come to thee in a thick cloud, so that you should draw near to the thick darkness in order that the people may hear when I speak. They themselves will be prophets when I speak, not [necessitating] that it should be confirmed to them through others,” just as it is said, When the Eternal said unto me: Assemble Me the people, and I will make them hear My words, that they may learn to fear Me all the days. “And they may also believe in thee forever, i.e., through all the generations. And if there will arise among them a prophet or dreamer of dreams, 13:2. to refute your words, they will deny him at once, for they have already seen with their own eyes and heard with their own ears that you have reached the highest stage in prophecy. Through you, will become clear to them that which is written: If there be a prophet among you, I the Eternal make Myself known unto him in a vision, I do speak with him in a dream. My servant Moses is not so; he is trusted in all My house; with him do I speak mouth to mouth.” It is for this reason that He said, “in order that the people may hear when I speak with thee, for they will hear when I speak out of the midst of the fire, and they will know that it is I the Eternal Who speaks to you. They will believe in My words and also in you forever.” Similarly, that which the people said, This day we have seen that G-d doth speak with man, and he liveth, was to state: “Now the thing has been confirmed to us with the sight of our own eyes, as was the wish of G-d [that we should all become prophets]. Henceforth, Go thou near,, Verse 24. for we know that you have reached the great stage in prophecy, and hear all that the Eternal our G-d may say… and we will hear it from you, and do it,, Verse 24. for your prophecy has been confirmed to be above that of all prophets.” Now I have seen the Mechilta where it is written: “In order that the people may hear when I speak with thee. This teaches that the Holy One, blessed be He, said to Moses: ‘Behold, I will call you from the top of the mountain, and you will come up,’ as it is said, and the Eternal called Moses to the top of the mount, and Moses went up. And they may also believe in thee forever, i.e., ‘in you and also in the prophets who are destined to arise after you.’” The words of the Mechilta incline towards the opinion of Rabbi Abraham ibn Ezra.