http://revach.net/article.php?id=2181

Section: Halacha   Category: Tshuvos
Rav Shlomo Zalman Auerbach: Tefila Over Loudspeakers At a Big Gatherings
Rav Shlomo Zalman says that hearing something through an electronic device such as loudspeaker or telephone is not considered hearing in Halacha.  The reason is that you are not hearing a person’s voice rather you are listening to an electronic simulation of it. Technically you cannot be yotzei any bracha or even answer amen.

If so during large gatherings where a microphone is used by the Chazan, how could people answer amen?  Rav Shlomo Zalman explains that the shul in Alexandria, Mitzrayim was so large that people could not hear the Chazan.  They set up a flag system where people would answer upon seeing the different flags being waived and this way would be able to participate in the tefila and they answered amen even if they didn’t actually hear.

Similarly he says if you are standing in a minyan and cannot hear the chazzan, but only know that amen is being said because you hear it on the speaker system, then you may answer amen together with the Tzibbur.  Not because you are answering to the loudspeaker but rather because you are part of a minyan.  Although you do not technically hear what they are saying, the loudspeaker indicates this to you.  However if you are not standing in the presence of the minyan you may not answer. (Halichos Shlomo, Tefila 22:15)

Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.