The one "no man understands him" would the people speaking in tongues. The 120 had no idea what they were saying, but the ones standing near by did.
What was being said to these people were "the wonderful works of God" which would have been "mysteries" that they did not know.
Act 2:11 Cretes and Arabians, we do hear them speak in our tongues the wonderful works of God.
Sorry, in this case (Pentecost) some men DID understand them and in Paul's later reference where he uses this phrase he is by the phrase speaking about those who hear not those who speak (of course they have no idea). And as for the "mysteries" Paul is clearly referring to the speaker speaking mysteries TO GOD (a mystery to the speaker speaking TO Him, but not a mystery to God) while at Pentecost, which you yourself have so kindly indicated, they spoke the wonderful works of God to these people (God speaking through them....thus not the same).