The Zohar Hakadosh writes that the concept of speech was in exile in Egypt. There are thoughts, speech and action. The concept of speech is to express our loftiest thoughts into the world. It comes from a perfect marriage between body and soul. The greater the persons thoughts, the harder it is to express and limit them into finite words. Moshe Rabbeinu had a speech impediment precisely because of his greatness. He couldn't communicate his lofty ideas into speech. Babies cannot speak - not because of a lack of ability, rather, a lack of what to say! The child hasn't yet grasped concepts at a level that they can be expressed. Medrash explains that the Torah healed Moshes speech. Torah is the great gift that we were given which allowed us to truly speak. It comes from a lofty worlds but has words that we can use to express them into the word. In Egypt we couldn't fully express our loftiness. We were redeemed and received Torah and now true speech has been born.