May one answer Amen, Kaddish or Kedusha through a microphone?
Kaddish and Kedusha: One may answer Amen to Kaddish and answer Kedusha even if he does not hear the natural voice of the Chazan and only hears it through a microphone system. Nevertheless, there must be at least a Minyan of people who can hear the natural voice of the Chazan for the Minyan to be able to be valid.
Chazaras Hashatz: One is to initially try to hear the actual voice of the Chazan during Chazaras Hashatz. If this is not possible, one may answer Amen even to these blessings, so long as one knows which blessing he is answering Amen towards.
 Conclusion of Admur 124/11 that one is to be stringent like both opinions and according to the first opinion one is not Yotzei if he does not hear the blessing from the person and he thus may not answer Amen due to it being an Amen Yesoma.