The holy Zohar describes the great reward for one who is a Shomer Emunim (lit. “keeper of the faith” [cf. Yeshayah 26:2]) — that is, he sits in anticipation for the opportunity when a blessing will be recited and he will be able to respond “Amen!” Conversely, the punishment is very severe for one who does not attempt to answer amen.
One must therefore be very careful to recite one’s blessings out loud when he knows that there are people present who wish to answer amen. Then, the person who recited the blessing will also receive greater reward for it, since there is great reward for someone who brings merit to others. And anyone who causes his fellow to fulfill a mitzvah is considered as though he himself has fulfilled the mitzvah. And since “one who listens is as though he speaks it” (שומע כעונה), the person responding amen should say it aloud and bear in mind that he wishes to also discharge the obligation of the one who recited the blessing; in this way the person who said the blessing can hear the ‘amen’ and it will be considered as though he, too, said ‘amen.’ Together, the two of them will cause the Name of Hashem to be exalted.