Acts 2:5

Now there were dwelling at Jerusalem Jews, devout men from every nation under heaven. And when this sound was heard, the multitude came together, and were confounded, because that every man heard them speaking in his own language.

Heard them speaking in his own language ... Some have understood the miracle to have been in the hearers, as in Harrison's comment:

This is not the language of religious ecstasy. By a miracle, the language of the apostles was translated by the Holy Spirit into many diverse languages without a human translator. This phenomenon is not the same as the glossolalia, or gift of tongues, in 1 Corinthians 14, which were unintelligible until interpreted.[19]

It is certain, however, that the miracle was not in the hearers, but in the speakers. If the miracle is understood as being in the hearers, there would have been no need for a plurality of speakers; yet it is clear that all the apostles were speakers; the people "heard THEM speaking." Thus the wonder was not in the hearers, but in the speakers. After all, it was THEY who had received the baptism of the Holy Spirit.