1 Corinthians 14:9 in the World English Bible
So also you, unless you uttered by the tongue words easy to understand, how would it be known what is spoken? For you would be speaking into the air.
Context
8 For if the trumpet give an uncertain sound, who shall prepare himself to the battle?
9 So likewise ye, except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air.
10 There are, it may be, so many kinds of voices in the world, and none of them is without signification.