Whose voice did the crowd hear speaking in their own languages? Was it a voice inside their heads, or was it the disciples speaking their languages? Scripture tells us plainly:
Acts 2:6 "the crowd came together, and were bewildered because each one of them was hearing them speak in his own language"
It was the disciples who were speaking in the languages of the crowd. There is no mention in of any kind of 'miracle of hearing' taking place in Acts 2. If there was this great miracle of automatic translation in the ears of the hearers Luke would have told us, not remain silent on such an important fact. Verse 4 says the Holy Spirit fell on the disciples giving them the miraculous ability of speaking other languages they hadn't learnt - "they began to speak in other tongues as the Spirit enabled them" - before any of the foreigners heard them. There is no mention of the Spirit falling on anyone else. Notice the plural tongues in that verse. The disciples spoke in multiple languages, not a single 'heavenly' language