I have attended church services where the person teaching God's Word asks a question, or asks for an "amen" or a "hallelujah" or a similar statement from the congregation.
I agree that a believer in a church congregation may speak in tongues silently to God just as 1 Cor 14:28 instructs.
1 Corinthians 14:28 But if there be no interpreter, let him keep silence in the church; and let him speak to himself, and to God.
The majority of Greek Scholars will tell you that it does not mean "without any sound" but rather to himself and to God. As in "not addressing the assembly but subdued in a quiet manner to himself and to God"
Logic should also tell you this also, without knowing the Greek, because you cannot SPEAK IN TONGUES without speaking with your tongue.
Therefore a believer who is speaking to themselves and to God
in tongues which requires speaking (you can't speak in tongues in your head without using your tongue) while others are also praying and speaking to themselves and to God at the same time in a prayer time or worship time is not out of order and to find fault with it using this verse in a legalistic manner like "ah HA... gotcha ya! You're in violation of the Law" is a misunderstanding of the intent of Paul's instructions.
But those that find fault with the idea of making any sound at all in a prayer meeting are also going to feel uncomfortable in a prayer meeting where everyone is making sounds to themselves and to God in English. They are not accustomed to such noisy prayer meetings. They prefer no sounds except for a prayer leader speaking from a microphone.
Pentecostals believe that there is freedom for the whole prayer meeting to be of people praying in a quiet manner with audible sounds to themselves and to God. You have to get really close to them to hear them but they are not being "silent" praying in their heads only. Pentecostals usually think that prayer is speaking out loud not quiet thinking. Pentecostals imagine the first church prayer meetings to have been noisy not quiet.
If you insist that the word silent in this verse is the main point and means no sound that is your choice. Others do not put the emphasis of the verse on the literal definition of silent being no noise but on "to themselves and to God and not addressing the church" because one must use their tongue to speak in tongues to themselves and to God.
So if you wonder why they pray to themselves and to God in tongues using sound it is because they interpret that scripture differently than you do. Instead of saying "How can they do that and not know they are violating the RULE?" You can now say "I understand that they do not interpret that as meaning "in their minds" but subdued as in a quieter private manner.
Who is correct? We will find out when that which is perfect is come who was right. Until then Pentecostals think they understand it and others find fault. We can agree to disagree and the fault finders can go to churches that don't believe tongues are real. There they will not witness any of these things they think are violations of the rules of tongues which they don't think is real anyway.