‘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.'
In Jerusalem lived Jews (‘devout men' is elsewhere used mainly to indicate true-born Jews, but what follows suggests that it also includes proselytes - see Acts 13:43) from all parts of the known world, and because of the feast there would also be many visiting ‘devout people' present. Those ‘from many nations' were especially there because they were ‘devout'. They were either Jews who had come a long way to the Feast and were temporarily dwelling in Jerusalem or Jews who had returned to Jerusalem to spend their last years in the holy city in order to be near God's earthly Dwellingplace. And large numbers of both would be gathered at the Temple for the Feast, as they brought their offerings of firstfruits and came together to worship.
‘From ever nation under heaven.' This is a typical exaggeration, not to be taken literally, intended to indicate the widespread nationalities of the Jews present in Jerusalem at this Feast. At the coming of the Holy Spirit it was as though the whole world were present, confirming its universal significance. Here in miniature was the fulfilment of God's promises in the prophets that His word would go out to all the world. This was then later to be maximised by actually going out into the whole world (Acts 1:8). Here the whole world had flowed to Jerusalem, which would be followed by the word of the Lord going out to the whole world (Isaiah 2:2-4).
When they ‘heard the sound (phone)' they came to the spot where it had occurred, and was possibly still occurring. ‘The sound' probably indicates the wind (although in Acts 2:2 it is echos), but many commentators argue for it meaning the words in tongues. Either is possible. However, if the howl of the wind was heard in the Temple courtyards it would certainly be seen as so unusual as to draw a crowd, whereas the babble of voices would probably be lost among the continual babble of noise emanating from surrounding crowds, and the continual noise of the traders (John 2:13-16). But when they then saw how the disciples, whom they knew to be Galileans, were behaving, they gathered round, totally astonished to hear them speaking in many different languages, among which they recognised their own.