What was Hashem referring to when He said to Moshe "Mi Sam Peh ... "?
Rashi: Hashem was referring to the occasion when Moshe was captured and taken before Pharaoh to be judged. He was reminding him that it was He, Hashem, who taught him then to argue his case before Pharaoh,
Da'as Zekenim (verse 10): Moshe said that if someone by Pharaoh will speak to Moshe in one of the 70 languages, and Moshe cannot answer, they will mock him - whose Shali'ach are you?! Hashem answered, I enabled Adam ha'Rishon to call names to all creations, in all 70 languages. All the more so I will do so for you, to redeem My children!
Hadar Zekenim: I create a new mouth. All the more so I can fix yours! 3
Adam ha'Rishon (Targum Yonasan).
Da'as Zekenim (verse 10, citing R. Ovadyah): Hashem intimated that He was perfectly capable of granting him the power of speech at any time. Consequently, Moshe ought to have understood that if Hashem sends him on a Sh'lichus, He will provide him with the tools to carry it out (as this Pasuk explains).
Hadar Zekenim: The Midrash says that Moshe [when he was a baby,] he took a coal and was burned on his tongue (therefore, he could not speak well).
What did Hashem mean when He added " ... O Ilem, O Cheresh, O Pike'ach, O Iver?"
Rashi: It was He who rendered Pharaoh dumb - incapable of issue the orders to execute him; his servants deaf - unable to hear the command to execute him; 1 the executioner blind - who did therefore not see when he fled from the platform and escaped.
Gur Aryeh explains why Moshe needed all of these, even though Rashi said that Hashem gave Moshe eloquence to plead his case.
Why did Hashem say "O Mi Yasum Ileim", bearing in mind that dumbness is the lack of speech, and not something positive?
Ramban #1 (citing the Moreh Nevuchim #1): The Pasuk is referring to the (missing) word "Adam", meaning 'Who created a man dumb?'
Ramban #2: Hashem has given man a soul of speech, and since a dumb person has a stoppage in the nerves of the tongue, 1 the term "Yasum Ileim" is applicable.
Ramban #3 (Moreh Nevuchim #2): Since dumbness is the lack of speech, it is acceptable to write Yasum Ileim". 2
Rashi explains that Hashem rendered Pharaoh dumb, his servants deaf, and the executioner blind. He did not explain why it says Pike'ach!
Mizrachi: Hashem discusses a matter and its opposite, e.g. He gives a mouth [to speak] and makes mute. Pike'ach is the opposite of blind and deaf. Therefore it is put between them.
Da'as Zekenim (2:15): He made Moshe clever to escape.
Rashi explains that Hashem rendered Pharaoh dumb, his servants deaf, and the executioner blind, so Moshe could escape. Above, he said that the executioner's sword could not harm him! Refer to 2:15:1:1.
Riva: They thought that Moshe saved himself from the sword through witchcraft, and they intended to try killing him in another way.
Everything is a matter and its opposite, except for deaf - it does not say 'or hears!'
Riva: Also one who can hear is called Pike'ach. Refer to 4:11:151:1.