The worshiper who would pray in the assembly must do more than bow his head and close his eyes; he must pray. "Otherwise, if you bless with the spirit, how will he who occupies the place of the uninformed say "Amen" at your giving of thanks, since he does not understand what you say?" (I Corinthians 14:16). This verse suggests [that thee are] four requirements, if one is to enter into prayer:
One cannot legitimately say "Amen" at the conclusion of a prayer if he has not listened to the prayer. Mind-wandering is an ever-present problem. We sing, but we don't observe the words of the song. We bow our heads, but we don't listen to the prayer. We sit through the sermon, but our minds wander to things of an earthly nature. Consequently, we attend worship periods, but we don't worship as we ought. If one is to pray with the congregation, he must listen to the prayer.
When a man in the first century led a prayer in an unknown tongue, the worshiper could not say "Amen", for he could not understand the language in which the prayer was spoken. Neither can tile worshiper say "Amen" today, if the leader has not spoken loudly enough to be heard, or if he has used words or phrases which the worshiper does not understand. Those who lead prayers in the assembly can understand.
A number of years ago, while sitting beside an old preacher, I observed him saying "Yes" or ''Yes, Lord" at the conclusion of each separate phrase of the prayer as it was being led. He spoke the words softly enough that I was probably the only one in the assembly who could hear them, but I was impressed. Obviously, this brother was listening to every phrase, determining whether or not he agreed with the phrase, and was softly speaking his agreement. He was not just sitting through a prayer; he was [actively] praying. Occasionally, we hear sentiments expressed in prayer with which we do not agree. To these sentiments we cannot say "Amen".
The word amen means "so let it be". We long to hear the strong, resounding "Amen" at the close of prayers which we used to hear. We fear that the move away from this practice is just another step toward cold, lifeless formality in our worship periods. We are not contending, however, that one must say the word amen audibly; but we are suggesting that at least in his mind he should say "Amen", thUS making the prayer his own prayer. He has listened to the prayer; he has understood the prayer; he has agreed with the prayer; now he speaks to God his "Amen" - or approval of the prayer - as his prayer. In this manner, he unites with other worshipers in common prayer to God.
---Bill Hall, Exhortations & Stuff, 5/22/15 Whit Sasser, Ed. (Appleton, XII), www.appletonchurchofchrist.org