Luke speaks of "tongues of fire" in Acts 2:3 and "other tongues" in 2:4. These other tongues are enabled by the Holy Spirit. There is Jewish tradition for God speaking visibly, speaking from fire also. Recall Moses and the burning bush (Exodus 3). We also have Deuteronomy 4:36, "From heaven he made you hear his voice to discipline you. On earth he showed you his great fire, and you heard his words from out of the fire". (See also Habakkuk 2:1.) People could "see" God's voice. In Acts chapter 2 then, Luke really means some visible appearance of the voice of God manifesting to others through the disciples speaking in other tongues.
For reflection: Have I seen the voice of God? Have I experienced the fire of Holy Spirit?
Let us pray. "Summon your power, O God; show us your strength, O God, as you have done before. . . . Proclaim the power of God, whose majesty is over Israel, whose power in in the skies. You are awesome, O God, in your sanctuary; the God of Israel gives power and strength to his people. Praise be to God!" (Psalm 68:28-35).