With the phrase so with yourselves
(see also 1 Corinthians 14:12), Paul applies the imagery of 1 Corinthians 14:7–8 to the Corinthians’ own situation. The word tongue
in this context most probably refers to the speech organ. The tongue is an instrument of speech, just as the flute and harp are instruments of music. The question, How will anyone know what is said?
parallels the question in 1 Corinthians 14:7, How will anyone know what is played?
By speaking unintelligibly, the Corinthians are simply speaking into the air.
Instead of their speech reaching and transforming their audience, it simply disappears and the effort goes to waste.
9 So with yourselves, if with your tongue you utter speech that is not intelligible, how will anyone know what is said? For you will be speaking into the air.