What does 1 Corinthians 14:9 mean?
In the previous two verses, Paul has compared speaking in tongues without an interpreter to musical instruments that produce sounds or notes in no sensible order: with no melody. They create noise but no music. They are not guided by or received by a human mind in any meaningful way.Now he drives home the point to those speaking in tongues in the Corinthian worship services without any interpretation. If your speech can not be understood by anyone present, nobody will know what is said. You will only be "speaking into the air," meaning launching sounds that nobody will receive, making the whole exercise meaningless.
Again, Paul is not dismissing tongues as fakery and performance. He himself spoke in tongues at times. He reinforced the idea in this letter, especially, that speaking in diverse languages by the gift of the Holy Spirit was a positive experience, at least for the speaker. He wrote that he wished all the Corinthian Christians would receive this gift (1 Corinthians 14:5).
What he is condemning is the use of the gift in a church or public setting without interpretation by the speaker or someone else.