‘ “In the law it is written, With men of other tongues and other lips will I speak unto this people; and yet for all that will they not hear me, saith the Lord. Wherefore tongues are for a sign . . . to them that believe not.” 1 Cor. 14:21, 22. On the day of Pentecost “devout men, out of every nation under heaven . . . came together and were confounded because that every man heard them speak in his own language. And they were all amazed and marveled, saying one to another, Behold, are not all these which speak Galilaeans? And how hear we every man in our own tongue, wherein we were born? . . . We do hear them speak in our tongus the wonderful works of God.” Acts 2:5-11. They had not yet heard Peter’s preaching to them. He preached to them later in their own language. But they had overheard  the disciples as they were filled with the Spirit and spoke with other tongues were a most convincing sign to unbelievers. There have been many occasions since when this has happened, for tongues are set “for a sign.” ‘
Source: Ralph M. Riggs, The Spirit Himself (1949), Chapter 22: Tongues and Interpretation of Tongues, p. 164.
 [Note by crosstheology:] Why the author uses the word “overheard” here, is not clear to the author of this website. The Biblical account does not seem to imply this.