"All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them. Now there were staying in Jerusalem God-fearing Jews from every nation under heaven. When they heard this sound, a crowd came together in bewilderment, because each one heard them speaking in his own language." (Acts 2:4-6)
Was the miracle that the apostles were speaking in different languages or that the people heard them speaking in different languages?
R.C. Sproul (from whom I heard this) favors the latter and made the analogy of what happens at the United Nations: all the people from different countries hear the speaker in their own language.