3rd hour of the day (9am) was the 1st official hour of prayer. Having travelled a long way for the feast, Jews from all nations would have been in the Temple at that time, God, knowing this and wanting them to be rewarded for their faithfulness must have caused the tongues to be heard at the Temple, remember, trhe Temple had high and thick walls . . .fulfilling the prophecy "mine house shall be called an house of prayer for all people" (Isaiah 56:7 even "the sons of the stranger", v6), tongues is prayer not preaching. After the ascension they "were continually in the temple" (Luke 24:53, and indeed after! - Acts 3v1)
