G‑d spoke to Moses saying, "Come speak to Pharaoh, king of Egypt, that he send the Children of Israel from His land.' Moses spoke before G‑d saying, 'But the Children of Israel have not listened to me, so how will Pharaoh listen to me? And I have sealed lips."(Ex. 6:10-12)

What did he mean by, "I have sealed lips?" Earlier on [in the previous Torah reading] it was written, "I [Moses] am not a man of words… for I am heavy of mouth and heavy of speech". (Ex. 4:10) But the Holy blessed One replied, "Who makes a mouth for man? …I shall be with your mouth…." (ibid 4:11-12)

Why did Moses insist that Pharaoh would not listen and that his lips were sealed?

Now can one imagine that this did not happen, and that his lips were still sealed? Where then was the promise the Holy blessed One made to him?

Malchut was still in exile….

However, this is the secret: Moses [was granted] voice [when G‑d told him, "I will be with your mouth"], but speech as expression in words [referring to malchut1], was [still] in exile. He was unable to express [himself in] words. And for this reason he said, "How will Pharaoh listen to me? As long as words are in exile, I have no way to express myself. I [am on the level of] voice, but I am missing words, which are in exile." So the Holy Blessed One G‑d partnered him with Aaron.

The verse later states, "He shall speak for you to the people". (ibid. 4:16) Aaron was a prophet, and he was able to make the Divine presence - the Shechinah - manifest. By joining forces with Moses they would be able to make divine speech manifest to the Children of Israel. (Based on the Ramaz)

Moses was voice without speech….

Come and see: Throughout the time that speech was in exile, voice was disconnected from it, and speech was sealed up without a voice. But when Moses came [on the scene] voice arrived. But Moses was voice without speech, for speech was in exile. And as long as speech was in exile, Moses went as voice without speech [until he partnered with Aaron. But as far as Moses himself] - this situation continued until they came to Mount Sinai, when the Torah was given. At that time voice became joined with speech, and speech was expressed in words. And so it is written, "And G‑d spoke all of these words, saying…." (Ex. 20:1)

The explanation is as follows: the first instance of revelation of G‑d speaking to the Children of Israel was at Mount Sinai, when they heard the Ten Commandments. The name of G‑d used in the verse introducing the Ten Commandments ("And G‑d spoke all these words…") is the name Elo-him, spelled alef, lamed, hei, yud, mem. During the exile, the yud and the hei were separated from the name Elo-him, leaving only the letters alef, lamed, mem. which spell the word "elem", meaning "mute". But when the yud and the hei were combined with the alef, lamed, mem, forming the name Elo-him, then Divine Speech was heard. It was at that time that Moses was given the ability to express voice in speech:

Then Moses was whole as was fitting, [since] voice and speech were one whole…

Come and see: throughout the time that Moses was in Egypt, he wanted to extract speech from exile, but speech did not utter a word. When he left exile, and voice became joined to speech, the power of speech led Israel. But He did not actually speak [i.e. G‑d did not actually speak through Moses] until they came to Mount Sinai, when he began to speak words of Torah, which is the way it should be.

[Zohar I, p. 24b; translation and commentary by Moshe Miller
First published by Fiftieth Gate Publications and Seminars]