The Talmud speaks of an “Amen Yetomah” or “Orphaned Amen”. According to Rashi and Tosafot, this refers to a response of Amen made despite not hearing the berakhah. This is not a contradiction to the situation in the large synagogue in Alexandria (Sukkah 51b) where acoustical impossibility was addressed by having flags waved at the appropriate spots to indicate that Amen should be said. In that case, those answering knew they were responding to a berakhah and to which one they were responding, even though they cound not hear the actual voice. Rabbeinu Yonah cites this approach as well, but adds another one: in Alexandria, the ones answering had already prayed and fulfilled their obligations; in the problematic case, there is a desire to fulfill the obligation of the berakhah through the answering of Amen.

Rav Soloveitchik (Reshimat Shiurim to Sh’vuot 36b, p 116), posited that the issue here may be the question of what happens when one answers Amen: does it create a connection between the listener and the speaker, or does the word Amen it and of itself become infused with the essence of the berakhah. The suggestion of Rabbein Yonah seems to assume that the listener is joining with the berakhah itself, and thus must hear it personally. Rashi and Tosafot, however, understand the word itself to contain the meaning of the berakhah, and thus awareness of its content would suffice. (See also R. Yosef Engel, Gilyonei HaShas, who discusses an understanding of Amen Yetomah, based on the Yerushalmi, as an Amen answered without knowledge of the matters that one is affirming).