They began to speak in tongues as the Spirit gave them utterance. (Holy Spirit Class - 1967 - Session - 2)

Acts 2:1-4 KJV
And when the day of Pentecost was fully come, they were all with one accord in one place.
And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting.
And there appeared unto them cloven tongues like as of fire, and it sat upon each of them.
And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.

Rotherham
And, when the day of pentecost was filling up the number of days they were all together with one intent;—
When there came suddenly out of heaven a sound, just as of a mighty rushing wind,and it filled all the house where they were sitting;
And there appeared unto them—parting asunder—tongues like as of fire, and it sat upon each one of them;
And they were all filled with Holy Spirit, and began to be speaking with other kinds of tongues just as the Spirit was giving unto them to be sounding forth.