Notice Paul is writing from the perspective of the one doing the speaking. Paul says that from the speaker’s perspective, his own speech is unknowable and a mystery. This is true because when a person speaks in tongues under the influence of the Holy Spirit, he will speak in a normal,
understandable human language, though it is a language the speaker himself does not understand. That is the essence of the miracle of speaking in tongues.
Read further down in
Corinthians
1 Corinthians 14
6 Now, brothers, if I come to you speaking in tongues, how will I benefit you, unless I bring you some revelation or knowledge or prophecy or teaching?
7 Even in the case of lifeless instruments, such as the flute or harp, how will anyone recognize the tune they are playing unless the notes are distinct?
8 Again, if the trumpet sounds a muffled call, who will prepare for battle?
9 So it is with you. Unless you speak
intelligible words with your tongue, how will anyone know what you are saying? You will just be speaking into the air.
Intelligible - able to be understood; comprehensible.