@pjdann I certainly agree with your reservations about effectiveness - sounds too much like a magic pill To be clear, a subliminal message is a message that our mind perceives even though we are not consciously aware of it. Listening to YouTube while asleep may qualify, since we are not awake to hear it.
What exactly are they supposed to listen to? Is it a voice saying ‘you will grow taller’ over and over again or something like that?
I don’t think listening to something while you are asleep is inherently wrong, but I think the content should be godly and not based in superstition. For example, some people think you can speak things into existence - that our words have some sort of power. If you believed that, you might believe that having a word spoken over you again and again overnight would somehow make you taller. But the truth of Scripture is that we cannot manipulate the universe that way. If we need something, we should come to God and ask - not get involved in superstitious practices.
Some verses that may be applicable are when Jesus teaches us not to worry and not to swear. Why? Because if we have a concern we ought to bring it to God - not try to manipulate / alter things that are out of our control through superstition.
Matthew 5:25-27 - “Therefore I tell you, do not worry about your life, what you will eat or drink; or about your body, what you will wear. Is not life more than food, and the body more than clothes? 26 Look at the birds of the air; they do not sow or reap or store away in barns, and yet your heavenly Father feeds them. Are you not much more valuable than they? 27 Can any one of you by worrying add a single hour to your life?
Matthew 5:33-37 - “Again, you have heard that it was said to the people long ago, ‘Do not break your oath, but fulfill to the Lord the vows you have made.’ 34 But I tell you, do not swear an oath at all: either by heaven, for it is God’s throne; 35 or by the earth, for it is his footstool; or by Jerusalem, for it is the city of the Great King. 36 And do not swear by your head, for you cannot make even one hair white or black. 37 All you need to say is simply ‘Yes’ or ‘No’; anything beyond this comes from the evil one.
Matthew 5:6-8 - But when you pray, go into your room, close the door and pray to your Father, who is unseen. Then your Father, who sees what is done in secret, will reward you. 7 And when you pray, do not keep on babbling like pagans, for they think they will be heard because of their many words. 8 Do not be like them, for your Father knows what you need before you ask him.
Hope those thoughts are helpful. Christ grant you wisdom