Moshe Rabbenu says to Hashem in the beginning of Vaerah, הן בני ישראל לא שמעו אלי ואיך ישמעני פרעה ואני ערל שפתים. Why is Moshe saying he can't talk due to his speech impediment if he already said in Parshat Shemos לא איש דברים אנכי גם מתמול גם משלשם גם מאז דברך אל עבדך כי כבד פה וכבד לשון אנכי and Hashem responded,ועתה לך ואנכי אהיה עם פיך והוריתיך אשר תדבר ויאמר ה אליו מי שם פה לאדם או מי ישום אלם או חרש או פקח או עור הלא אנכי ה, Hashem already responded that he Moshe's lack of proper speech would not be an issue? Furthermore, why did Moshe say in Shemos כבד פה וכבד לשון and here he says ערל שפתים? And what is the ק"ו if Klal Yisrael won't listen, for sure Pharaoh won't, he doesn't have the קוצר רוח?
The Zohar Vaerah (25b) asks the question of what is Moshe asking about now and answers (translated into Hebrew) אלא סוד הוא; משה - קול, והדיבור, שהוא המלה שלו, היה בגלות, והיה הוא סתום מלפרש דברים. ובשל כך אמר "ואיך ישמע אלי פרעה" - 'בעוד שהמילה שלי נמצאת בגלות, שהרי אין לי מילה, הרי אני קול בלי מילה, שהיא בגלות'. ועל כן שיתף הקב"ה את אהרן איתו. The dibbur was in galus in Egypt and therefore Moshe could not speak. How does the Zohar answer the question, it was the same golus as before and Hashem already told Moshe that he would not have a problem? And how does adding Aharon help if the dibbur is in golus?
One of the reasons Chazal give for the golus in Egypt is that Abraham Avenu said בַּמָּה אֵדַע כִּי אִירָשֶׁנָּה (Nedarim 32a.) Avraham misspoke and that led to the golus. The golus in Egypt started due to the chain of events of the lashan harah spoken by the brothers about Yosef. Again, a misuse of speech. And Moshe feels the golus in justified after he experiences the lashon harah of Dasan and Aviram where he sees speech is misappropriated. The Sfas Emes (5632) says that is what the Haggadah means אנוס על פי הדיבור. How was it אנוס, Yaakov wanted to go see Yosef? He cites the above Zohar and reads Yaakov was forced to fix up dibbur (a reference to Yosef, in his words ופי הדיבור הוא יוסף והמש"י, presumably because Yosef is yesod and the ברית הלשון והמעור מכוונים זה לזה.) The golus had to happen to fix the miscues in speech. The golus was into Egypt, the land controlled by פרעה which is the letters פה רע, a bad mouth. He was the epitome of bad speech.
The Midrash says וּבַיּוֹם שֶׁיָּרְדוּ לְמִצְרַיִם, בּוֹ בַּיּוֹם עָלוּ, וּבוֹ בַּיּוֹם יָצָא יוֹסֵף מִבֵּית הָאֲסוּרִין, לְכָךְ הַלַּיְלָה הַזֶּה שִׂמְחָה לְכָל יִשְׂרָאֵל, שֶׁנֶּאֱמַר: לֵיל שִׁמֻּרִים הוּא לַה. The same day of the start of the golus is the say of redemption. Hence, on the day of going into golus because of speech, is the day of the tikkun where we have a mitzvah to speak about the miracles. Hence Klal Yisrael leaves passing פי החירות, the mouth of freedom, now their mouths were opened. בְּצֵאת יִשְׂרָאֵל מִמִּצְרָיִם בֵּית יַעֲקֹב מֵעַם לֹעֵז, when we left Egypt we were redeemed from the nation of strange tongue, out mouths became opened. אנכי ה אלקיך הַ֖מַּֽעַלְךָ מֵאֶ֣רֶץ מִצְרָ֑יִם הַרְחֶב־פִּ֜֗יךָ וַֽאֲמַלְאֵֽהוּ, since you left Egypt your mouth is now open to receive divrei Torah (see Berachos 50a.) The Sefer Yetzirah says about the month of Nissan המליך אות ה' בשיחה כו. Nissan is the month of speech (the ה is connected with תורה שבעל פה, as Rav Tzaddok (Rosh Chodesh Nissan 1) elaborates about.)
Rav Tzaddok notes that the berachot were received by Yaakov on Pesach. The beracha of הקול קול יעקב, the power of the voice of Yaakov was established on Pesach. The ammunition of Klal Yisrael to stand through the golus is the power of speech which becomes established on Pesach.
When Hashem directs Moshe to speak it is not to speak empty words. The words of Moshe have an effect on the world. In order for that effect to be actualized there has to be a recipient. If the words fall on deaf ears they don't have any effect. The Be'ar Moshe explains the קל וחומר of Moshe. The sense of speech and hearing go hand and hand. One learns to talk by listening to others. He reads מקוצר רוח, the רוח as a reference to the רוח ממלא of a person, the power of speech was removed from Klal Yisrael and hence they did not have the power to listen to the message of Moshe Rabbenu. (The Midrash attributes the lack of Klal Yisrael listening to their sins, in other words it is not the work in a physical sense that made their ears dull to the message but they were so lost in the golus they could not envision Hashem taking them out, they were incapable of absorbing words of freedom.) The Sfas Emes (previous piece) continues, כמ"ש במ"א פי' זוה"ק הנ"ל שדיבור שהוא הכרת מלכותו ית' בכל הדברים. זה הי' נסתר בגלות. וע"י גאולת מצרים נודע גבורתו ית. The word for thing and speech in lashon hakodesh is דבר for all things are composed of the words of Hashem which is their life force (as explained in the Tanya Shaar Hayichud Ve'emunah,) and Klal Yisrael lost their ability to see the Divine in everything. They lost the understanding of dibbur and required the 10 maakot to bring it back.
The ק"ו of Moshe is Bnei Yisrael who are closer to Hashem are so lost in the golus that they can't hear the message of freedom, for sure פרעה, the פה רע himself, will not be able to absorb anything I am telling to him. This is what the Moshe is asking Hashem. Beforehand, my speech was כבד, it was difficult, but there was somewhere for the words to 'land' because Klal Yisrael may have absorbed the message. However, now, after Pharaoh said תכבד העבודה and removed the day of rest of Shabbas, that little bit of spiritual respite in which the ears of Klal Yisrael may be slightly opened, now I am totally ערל שפתיים, I am closed entirely, there is no room for words to be able to have an effect. The golus of the dibbur has totally removed my capabilities to say anything. Hashem says that I will provide with Aharon. Since Aharon is a man of the people, the אוהב שלום ורודף שלום, he is able to provide the people with a message of Hashem that will uplift them (based upon Rav Tzaddok Vaerah 6,7.)
The Evan Era (14:4) says והנה המצורע והבית המנוגע וטמא מת קרובים, והנה גם הם כדמות פסח מצרים. The taharah process of the metzorah with the עץ ארז ושני תולעת ואזוב parallels the blood put with the עץ ערז on the doorpost and the bird is akin to the korban pesach. The tahara of the metzorah resembles what Klal Yisrael did at makkas bechorot. Why? Because the sin of the metzorah is for misusing his/her speech. They go into a mini golus like the golus of mitzraim to fix the problem and they need a mini yitzias mitzraim in order to rehabilitate their power of speech.