May one hear the Megillah through speakers; microphone or a broadcast system, such as the telephone, radio, or live internet hookup?
Initially, although there were some Poskim who understood that one may fulfill a Mitzvah through hearing it on a radio, telephone, microphone or broadcast system, practically after further research into the understanding of how these systems work, the Poskim conclude that this may not be done. Thus, one cannot be Yotzei the Mitzvah of Megillah reading through hearing the Megillah through a broadcast, radio, telephone, or live hookup. Likewise, one cannot be Yotzei through hearing the Megillah through a microphone or sound system. Due to this reason, a microphone is not to be used by the Megillah reader even if it is difficult for people sitting there to hear. If they cannot move closer than a second reading should take place on their behalf. In the event that one already heard the reading via radio and the like, he is to rehear it with a blessing. [If, however he heard it with a microphone, then he is to rehear it without a blessing.]
Answering Amen to the blessings: If one is listening to the Megillah reading from any of the above technologies for the sake of being Yotzei, then since he indeed is not Yotzei, it is forbidden for him to answer Amen to the blessings that he hears. Furthermore, if in the above cases there is even a slight delay of a few seconds between the real live reading and the broadcast that one is hearing, then one may not answer Amen to the blessings even if he is not hearing it for the sake of being Yotzei, due to it being an Amen Yesoma.
One cannot be Yotzei Megillah reading through hearing the Megillah through a microphone, sound system, radio, telephone or live hookup. One may not even answer Amen to the blessings said before and after the Megillah.
 Minchas Elazar 2:72; Rav Pesach Tzevi Frank in Mikraei Kodesh 11 based on Halachos Ketanos 2:45; Hearos on Halachos Ketanos 276; Possibility entertained by Igros Moshe 2:108; 4:126 regarding Bedieved or time of need [See there that his leaning opinion is to permit being Yotzei and that so should be done in a case of need, that one is unable to hear Havdala or Megillah otherwise, and that perhaps one is not to protest one who is lenient, although he concludes to protest being that the allowance is unclear]; Tzitz Eliezer 8:11-10 regarding Bedieved or time of need; Sheivet Halevi 5:84 that from the letter of the law seemingly one is Yotzei [Rav Yaakov Meir Shtern claimed that the Shevet Halevi was indeed lenient to rely on this Bedieved ]; Opinion of Rav Shteinman quoted in Vayishma Moshe Gilyon 23
The opinion of the Chazon Ish: Chazon Ish, brought by Rav SZ”A in footnote in Minchas Shlomo 1:9, replied to Rav SZ”A with an argument to be lenient to validate if one heard a Mitzvah from a microphone, radio and the like, although Rav SZ”A concluded that he does not understand his argument; Practically, Rav Chaim Kanievsky stated that the Chazon Ish was not lenient to permit hearing Megillah from a microphone and even Bedieved one is required to re-hear it
 The reason for the allowance: As perhaps the above machines send the actual voice of the reader to the ears of the listener. [Igros Moshe ibid] Furthermore, even if the transmitted voice is not the natural voice of the reader, perhaps there is no need to hear the actual voice so long as the voice that one hears is the direct result of the actual voice and is heard at the same time as his natural voice. Furthermore, perhaps all hearing is through the sounds being reproduced in the air until it reaches the ear of the listener, and nonetheless one is Yotzei. [Arguments of Chazon Ish ibid; Igros Moshe ibid; Shevet Halevi ibid; Mikraeiy Kodesh ibid that so is proven from Halachos Ketanos ibid who permits mechanical hearing aids as carrying a sound through a secondary means is valid]
 Daas Torah 689 based on Halachos Ketanos; Mahraiy Engel Brachos 25; Rav SZ”A in Minchas Shlomo 1:9 “One cannot be Yotzei anything from a microphone and speakers or the radio and telephone, and those who puzzle at this ruling simply do not understand the mechanics of these machines, thinking erroneously that there is some kind of magical transportation of the persons actual voice. I have lengthened in this Teshuvah to negate this mistake, as they do not know the truth, and I have spoken with people knowledgeable of this matter, and they all agree with me.”; Eretz Tzvi 1:23; Mishpitei Uziel 1:5; Final ruling of Chazon Ish, as brought in previous footnotes; Minchas Yitzchak 1:37 and 3:38; Yechaveh Daas 2:68; 3:54; Chazon Ovadia Megillah 56; Mishneh Halachos 4:85; Kinyan Torah 1:75; Beir Moshe 3:166-168; Piskeiy Teshuvos 689:3; Rebbe in Igros Kodesh 23:304 and Likkutei Sichos 21:496, printed in Shulchan Menachem 3:319,“What is heard is not a man’s voice at all (and does not even resemble the case of one who sounds a Shofar in a pit). It is obviously most novel, even strange, to suggest that an indirect effect (koach kocho) and a basic transformation of the very nature of speech, should serve as a substitute for speech. Even though the recent scholars who have ruled otherwise include reputed poskim, it is evident from their very responsa that those who explained them the nature and workings of the telephone made a basic mistake.”; Rav Yaakov Yosef Z”l completely invalidates using it, and states the Poskim who permitted made a mistake in their understanding of these systems; The following lenient Poskim all agree that one is initially to be stringent: Igros Moshe 2:108; 4:126; Tzitz Eliezer 8:11; Sheivet Halevi 5:84 that it is forbidden to hear from the above systems even though it is possible that one is Yotzei
Other opinions who rule that despite the new and confirmed knowledge of the system one can be Yotzei: Some of the lenient Poskim ibid rule that despite the confirmed understanding of the mechanics of the above machines and that the sound is not the natural voice, nevertheless one is still Yotzei, or is at least questionably Yotzei the Mitzvah, as perhaps there is no need to hear the actual voice so long as the voice that one hears is the direct result of the actual voice and is heard at the same time as his natural voice. [See Chazon Ish ibid; Igros Moshe ibid; Mikraeiy Kodesh ibid; Tzitz Eliezer ibid]
 The reason: As one must hear the readers voice to be Yotzei the Mitzvah [Admur 124:11], and this voice must be the persons natural voice [See Admur 587:1; Michaber 587:1; Gemara R”H 27b where echoes are invalidated for Shofar hearing] and one is thus not Yotzei the Mitzvah if he hears an artificial mimic of the readers voice. Now, being that the above technologies do not magically send the natural voice of the reader to a further distance but simply make an artificial mimicking of it, therefore one cannot be Yotzei the Mitzvah through hearing the Megillah through a broadcast, radio, telephone, or live hookup. Just as one cannot be Yotzei a Mitzvah by playing a recording of the Mitzvah, as one is not presently hearing the voice of one who is obligated in the Mitzvah, but rather of a machine which contains no obligation, so too one cannot be Yotzei even if the event is live, as in both cases it is a machine made voice being sounded, and machines are not obligated in Mitzvos. [Rav SZ”A ibid; Rebbe ibid] This is in addition to the fact that some Poskim rule that it is forbidden to ever answer Amen through a distanced transmission being that there are churches and bathrooms in the interim area where the voice travels. [See Piskeiy Teshuvah 167; Rav SZ”A in Minchas Shlomo 9:1; Moadim Uzmanim 6:105; Mishpitei Uziel 1:5 [brought in Igros Kodesh 13:221 as opinion of Sefaradim]; Beir Moshe 3:166-168]
 The reason: A) It is not to be used as those who are close enough to hear the natural voice are anyways Yotzei and those who are too far and are only hearing the microphone voice are anyways not Yotzei, so using the microphone just confuses the listeners as to whether they could actually hear or not. B) Although, the microphone does note invalidate the hearing for those who are close enough to still hear the natural voice being that the hearing of two readers is valid by Megillah reading [see Michaber 690:2] Nonetheless, this only applies if one in truth is able to concentrate and hear every word, otherwise he does not fulfill his obligation. [M”B 690:4; Kaf Hachaim 690:10]
Other opinions: See Chazon Ovadia ibid and Yalkut Yosef Moadim p. 287 that if the congregation can hear the natural voice of the reader then a microphone may be used to simply elevate the sound of his voice. Practically, although some Sepharadi communities follow this practice, it is not to be done due to the reasons explained above. One who heard the Megillah from such a reading is to rehear it without a blessing unless they are certain that they heard and paid attention to the natural voice of the reader.
 Poskim ibid; However, see Shevet Halevi ibid that he is to rehear it without a blessing; So also rules Rav Shamaiy Gross, and other Rabbanim of today
 As perhaps one was able to hear the natural sound of the reader, despite the microphone. Nonetheless, if in truth it can be ascertained that one could not hear the natural sound without the microphone then he is to rehear it with a blessing.
 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; Chazon Ovadia ibid
Other opinions: Some Poskim rule that one is to always answer Amen to a blessing said over the telephone, due to doubt. [Igros Moshe 4:26]
 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.
 Admur 124:11; Rama 124:8; Abudarham; Brachos 47a; M”B 124:34