Another question I have had on the subject is whether or not those who believe they are speaking in tongues believe God to be controlling every muscle movement of their mouth?
Is every phoneme they utter exactly what God wants expressed? Or is there some percentage of human error in their phonemes?
Is God is specifically creating intelligible words and sentences in the person's mouth, or is something else happening?
I am not saying it is not happening. I just want to learn more about what is happening.
How can you tell the difference between a word God wants you to say and something your own subconscious brings up? Does God control your muscles, or does He give you images and your subconscious translates that into mouth muscle movements (the way we do with normal speaking)?
Prayer, discernment, experience... and testing.
Remember there are two kinds of tongues. One is prophecy with interpretation. That is an assembly event and normally reserved for leadership. At this point I don't think this is a concern for you are far as your question how asks, but an overall understanding of the 'process' helps. A church I went to had a rule that if someone in the assembly felt they were being led to prophesy, they should contact leadership. Same with someone feeling led to interpret. They all prayed for understanding and discernment, and if the consensus was that the Holy Spirit was indeed wanting to speak then they would be give the ok to speak when and as the Spirit led. While I don't find this exact template in the Bible, I do believe that it is of the need for order and not confusion as Paul spoke about in 1 Cor 14. Remember, Paul said he wants us all to do this, but that he'd rather we not do it than do it wrong and spread confusion. So that would be a test for an assembly, does the tongue elicit order or confusion?
Tongues in private prayer - more to your question and that's a hard one to describe. God doesn't give my mind the words and then I speak them (altho I do often have an idea of the subject of the prayer), nor does God control my lips. I physically move my lips, exhale and speak, but He controls the words that come out. I'm struggling on how to better describe it. It's one of those yielding in faith to the mysteries things. Kinda like Peter walking on the water. As long as Peter focused on Jesus he was ok. When his focus changed to the storm is when it went wrong. Focus on Jesus not your lips, and your lips will do what Jesus wants them to.
It might help to understand the origin of tongues. When man was created and as his numbers grew, he had a pure singular spiritual language. Man didn't lose that until the tower of Babel. Now, the purpose of the Messiah/Christ is to restore everything back to creation that man has lost. And one of those things He restores is that pure singular spiritual language. Jesus has indeed won the right to restore these things, but has yet to return to this world to enforce them. So He allows us to participate with Him thru the Holy Spirit to bring some of that restoration to the here and now, thru the gifts of the Holy Spirit.
For example, man was never meant to be sick. We lost eternal health at the fall. Jesus has the authority to restore that, and does so here and now thru the gift of healing.
Likewise, man was intended to have a singular language but lost it. Jesus has the authority to restore it, and He does so thru the gift of tongues.
In that sense, tongues are our native language, and we have to relearn it. Like you do in any language class, focus on the teacher and the lesson, and your lips will form the words as you speak without you having to think about it.