Silence means silence; so there is no uttering out loud this so called praying in tongues as that would be a distraction to those around them while trying to listen to the main event of 2 or 3 speaking in tongues one at a time, while another interpret.
Prophesy is to be done that way with 2 or 3, one by one, ( 1 Corinthians 14:31 ) and for another to judge and so it is the same for tongues. And since that is the same for tongues, why would Paul allow believers to speak in tongues privately out loud in the assembly even in a whispering mode? If someone came in unlearned and saw a bunch of believers speaking in tongues "privately", would he not deem them all mad? Paul said so. 1 Corinthians 14:23
Indeed, when someone is talking or whispering in church today, those besides them usually tell them to be quiet because they are trying to hear & listen.
So I believe what Paul is saying in 1 Corinthians 14:28 is about a foreigner speaking out of turn while this 2 or 3 speak in tongue and another interpret practice is going on in church because this foreign visitor doesn't know what is going on. It is because what he is saying is not coming with interpretation is why Paul is saying for him to be instructed to be quiet because he knows what he is saying as one speaking unto himself because that is what is meant by Paul and not how we would take it to mean as a crazy person talking to himself.
And the same goes for speaking unto God in that he is not really speaking to God but only God understands what he is saying in the real God's gift of tongues in 1 Corinthians 14:2 for why the tongue speaker is not speaking to himself because he does not know the language but God does even though in the spirit, he speaks mysteries. the fat hat there are no mysteries to God is why he is not speaking to God but as heard by himself in not knowing what was being manifested by the Holy Spirit.
So in context, 1 Corinthians 14:1-15, the gift of prophesy is the better gift to seek after over all spiritual gifts, including tongues because tongues is not a stand alone gift to use in the assembly for it is to come with interpretation for that to even be understood by the tongue speaker for that tongue to be actually fruitful to the tongue speaker.
So 1 Corinthians 14:28 was Paul saying that guy is not speaking in tongues because he understands what he is saying as God does, for why there is no interpretation and so that is why he is to be quiet in the assembly.