1 Corinthians 14:7-9

Amplified Bible

⁷ Yet even lifeless things, whether flute or harp, when producing a sound, if they do not produce distinct [musical] tones, how will anyone [listening] know what is piped or played? ⁸ And if the [war] bugle produces an indistinct sound, who will prepare himself for battle? ⁹ So it is with you, if you speak words [in an unknown tongue] that are not intelligible and clear, how will anyone understand what you are saying? You will be talking into the air [wasting your breath]!