1 Corinthians 14:9

So likewise you, unless ye utter by the tongue words easy to
be understood
This is an accommodation of the foregoing similes to the case in hand: for as unless there is a distinction of notes and tunes, it cannot be music, nor it cannot be known what is piped or harped; and unless the trumpet gives a certain sound, none can know when to prepare himself for the battle; so unless in the public ministry and service a language is spoken, and words made use of, which are understood without difficulty:

how shall it be known what is spoken?
the subject matter of the discourse, prayer, or psalm, will be all lost, and therefore cannot be for edification, exhortation, or comfort:

for ye shall speak into the air;
into which such words are resolved, and the use and benefit of them, as to others, cease with the breath, by which they are delivered: any part of divine service performed in such a way is all in vain, and to no purpose; it is all lost labour, it is beating the air, and talking to the wind. This condemns the practice of the Papists, performing divine service in a language not understood by the common people; and exposes the folly of those, who are fond of a florid style, of bombast words, great swelling words of vanity in their public discourses: this is only speaking into the air, with regard to the vulgar, whose edification should be consulted: and as the end of the Gospel ministry is public usefulness and edification, plainness of speech, words easy to be understood, should be used; such as are apt and fit to convey the true idea of things to people in common; these are the acceptable words, which the wise preacher, who is desirous of doing good to the souls of men, will seek out, and studiously make use of.