if he spoke in Paul he also spoke and taught them.
That is a non-sequitur. Just because Christ spoke in the apostle Paul, doesn't mean he also speaks in others. Christ spoke in and through apostles because they were his authorized spokesmen.
Luke 10:16 "“The one who listens to you listens to Me,"
2 Peter 3:2 "and the commandment of the Lord and Savior spoken by your apostles.
1 Cor 14:37 "let him recognize that the things which I write to you are the Lord’s commandment."
1 Thes 2:13 "when you received the word of God which you heard from us, you accepted it not as the word of men, but for what it really is, the word of God,"
1 Thes 4:15 "For this we say to you by the word of the Lord...."
In 2 Cor 13:3 the Corinthians were seeking proof that Christ was speaking in Paul
"Since ye seek a proof of Christ speaking in me....". And thus evidence he was an apostle, an authorized spokesman of Christ. If Christ was already speaking in all the Corinthians, how could Christ speaking in Paul be evidence of his apostleship?
When he says that Christ spoke in him and was mighty in them also. This is clearly saying that he spoke in them and through them also.
It says nothing of the sort. There is no "also". You are adding to scripture. It says:
"Since ye seek a proof of Christ speaking in me, which to you-ward is not weak, but is mighty in you.".
"Mighty in you" does not mean speak in you. Paul was referring to the demonstrations of miraculous power Christ had performed in them. Dunateó is the verb form of dunamis, the power to perform miracles (1 Cor 12:10), one of the many spiritual gifts which the Corinthians were well endowed. Paul says more about this power in the very next verse.
"For we also are weak in him, but we shall live with him by the power (dunamis) of God toward you." (2 Cor 13:4)
If Jesus Christ is in all believers do you not think he speaks in them and they to him? When you pray in your heart do you think you are talking to God? Or is this a fiction and you don't really think God can hear you and respond? God speaks in that still small voice in the heart. I have heard him many many many many times. This is the normal christian life for those who walk in the light of Christ.
God doesn't speak to us via inner feelings in our hearts. There is not a single verse that says that. He speaks to us by scripture - My sheep hear my voice. There numerous verses that tell us God's voice is scripture.
Psalm 103:20 "Mighty in strength, who perform His word, obeying the voice of His word!"
Judges 2:20 "So the anger of the Lord burned against Israel, and He said, “Because this nation has transgressed My covenant which I commanded their fathers and has not listened to my voice"
2 Kings 18:12 "because they did not obey the voice of the Lord their God, but transgressed His covenant, even all that Moses the servant of the Lord commanded;
Psalm 106:24-25 "They did not believe in His word, but grumbled in their tents; they did not listen to the voice of the Lord."
Jer 9:13 "The Lord said, “Because they have forsaken My law which I set before them, and have not obeyed My voice nor walked according to it".
Deut 13:18 "if you will listen to the voice of the Lord your God, keeping all His commandments"
Deut 15:5 "if only you listen obediently to the voice of the Lord your God, to observe carefully all this commandment"
Deut 26:14 "I have listened to the voice of the Lord my God; I have done according to all that You have commanded me."
Ex 19:5 "Now then, if you will indeed obey My voice and keep My covenant"
Ex 23:21 "But if you truly obey his voice and do all that I say, then I will be an enemy to your enemies and an adversary to your adversaries. " (this is Moses speaking)
Dan 9:10 "nor have we obeyed the voice of the Lord our God, to walk in His teachings which He set before us through His servants the prophets."
Daniel 9:11 "Indeed all Israel has transgressed Your law and turned aside, not obeying Your voice;"
1 Sam 12:14 "If you will fear the Lord and serve Him, and listen to His voice and not rebel against the command of the Lord"
1 Sam 12:14 "If you will not listen to the voice of the Lord, but rebel against the command of the Lord,"
Jer 11:3-4 "Thus says the Lord, the God of Israel, “Cursed is the man who does not heed the words of this covenant which I commanded your forefathers ..., saying, ‘Listen to My voice, and do according to all which I command you; so you shall be My people"
Jer 26:12-13 "The Lord sent me to prophesy against this house and against this city all the words that you have heard. Now therefore amend your ways and your deeds and obey the voice of the Lord your God".
Jer 40:2-3 "The Lord your God promised this calamity against this place; and the Lord has brought it on and done just as He promised. Because you people sinned against the Lord and did not listen to His voice, therefore this thing has happened to you".
Psalm 81:11 “But My people did not listen to My voice, And Israel did not obey Me.
"4 Abide in me, and I in you...7 If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you."(John 15:4,7 KJV)
Where Jesus is in us he speaks his word also in us. Paul showed that Christ spoke in him. This should be enough of an evidence.
Again your proof text says nothing of the sort. The verse says nothing about Jesus "speaking words in us". You do this all the time, quote a verse and then claim it says something completely different to what it actually says. That is the fallacy of eisegesis. The verse says "my words abide in you". Jesus was referring to the words he spoke to the disciples.