When the promise of the Father descended upon the disciples on the day of the Pentecost, Luke tells us that the apostles preached in foreign tongues, so that those who spoke other languages would be able to understand the Gospel (Acts 2:2-4 and 7).  However, as we shall see, "speaking in tongues" takes place mutely, inside the heart. In the passages that follow, the Apostle Paul clearly says that only the Christian who is "in the Spirit" hears those tongues, and that they are there, only for his own edification.  Besides, that is why they take place inside his heart with MUTE sighs: he would be able to voice them, but only if there were an interpreter present.

"....the Spirit of His Son within your hearts, crying out: 'Abba, the Father'..."   [Galatians 4:6]

"....But if the Spirit of the One Who raised Jesus dwells within you.... He shall give life.... through the dwelling inside you of His Spirit... this Spirit shall co-testify together with our spirit.... this Spirit intercedes for us with mute sighs.... the One Who searches the hearts knows what the conscience of the Spirit is: that It intercedes in favour of the saints..."   [Romans 8:11, 16, 26, 27]

"...but fill yourselves in the Spirit, speaking (λαλούντες) to yourselves in psalms and hymns and odes, singing and chanting psalms to the Lord in your heart, giving thanks always, for everything..."   [Ephesians 5:18-20]

"...for, the one who speaks (λαλεί)a tongue speaks (λαλεί) not to people, but to God, because no-one hears him, for his spirit speaks (λαλεί) mysteries. However, one who prophesies (προφητεύων) to people speaks (λαλεί) constructively... whereas the one who speaks (λαλεί) in tongues edifies himself only... 

And though I want all of you to speak (λαλείν) in tongues, I would rather that you prophesied (προφητεύητε); for he is greater who prophesies (προφητεύων), rather than the one who speaks (λαλών) in tongues; unless he is interpreting, so that the Church may receive edification... Because if you bless (minister, officiate) in the Spirit, then how shall they who are in the place of the laity respond with "Amen" when you are giving thanks? Because they will not know what you are saying... and let there be one who interprets. Otherwise, let him keep silence in church, and speak (λαλείτω) to himself and to God..." [1 Corinthians 14:2, 3, 14, 27, 28]

We believe that it is now abundantly clear from the above passages that "speaking in tongues" is something that takes place secretly inside the hearts of the sons of God. It is what the Orthodox Church today refers to as "the Prayer of the heart"  (1 Corinthians 14:14: "My spirit is praying...")