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Who is obligated in the Megillah reading? 
Women: This obligation applies equally to women. [For this reason the men are required to read the Megillah for the girls at home. However in many places the women are accustomed to go to Shul to hear Megillah. However this is only valid if they heard the reading properly, otherwise they must re-hear it another time.]
Children: Children [boys and girls, who have reached the age of Chinuch], are to be educated to read [or hear] the Megillah [once by night and once by day]. Regarding bringing children to Shul-see Halacha 7; Regarding reading the Megillah for a child that did not yet hear-see Halacha 11 Q&A!
Is a deaf person obligated to read the Megillah to himself?
If one can only hear with a hearing aid is he able to fulfill his obligation through listening to the Megillah reading?
Non-electric hearing aid: If he is able to hear with the help of a non-electric hearing aid then he is obligated in the Mitzvah. He may thus hear the Megillah from others and also read Megillah for others.
Electric hearing aid: One does not fulfill his obligation with hearing Megillah through an electric hearing aid.
If one is unable to hear without an electric hearing aid he is to read the Megillah to himself rather than hear the reading from others.
Is one with a hearing difficulty to remove his hearing aids prior to hearing Megillah?
As stated above, one does not fulfill his obligation with hearing Megillah through an electric hearing aid. Therefore if one is able to hear without the hearing aid, he must remove it prior to the reading in order for him to fulfill his obligation.
Sparks of Kabala
Why was every Jew obligated in the Megillah reading if many Jews were not under the threat of Haman?
The twelve tribes of the Jewish people correspond to the twelve combinations of the Tetragrammaton [Sheim Havayah]. The world cannot exist without the completeness of these twelve combinations. Thus although only the tribes of Yehuda and Binyamin were under the jurisdiction and threat of Haman, nevertheless their annihilation would have led to damage in the Divine name, which would lead to the destruction of the rest of the world. Thus all Jews must celebrate Purim even if they were not a direct threat of the decrees.
 As the verse states “Kiyimu Vikiblu” which includes even the converts that were with them. [Kaf Hachaim 689/4]
 It is disputed whether slaves that have not been freed are obligated in the Megillah reading as are women. [M”B 689/2] The Smag; Bach; M”A 689/2 and Peri Chadash 689/3 rule they are obligated while the Gr”a; Ateres Zekeinim rule they are exempt. Practically they are to read Megillah without a blessing, and hear the blessings from another person. [Kaf Hachaim 689/5; See also Biur Halacha 689 “Veavadim”] According to all a slave is unable to read the Megillah on behalf of another person. [Biur Halacha ibid]
 The reason: As also women were involved in the Purim miracle. [Tractate Megillah 4] This means to say as follows: Although the reading of the Megillah is a time dependent Mitzvah, nevertheless they are obligated in its reading, being that at first the decree of annihilation was made also against women. [M”B 689/1] Alternatively it is because the miracle was done through a woman, Esther. [Kaf Hachaim 689/2]
 M”A 689/1; M”B 689/1; See Tosafus Eiruchin 3a that Rebbe Yehoshua would read the Megillah for his household. [Kaf Hachaim 689/3]
 P”M 689 A”A 1; Kaf Hachaim 689/3; M”B 689/1
 M”B 689/3
 M”B 689/3; Kaf Hachaim 689/6; This is the age that the child understands the meaning of the Purim miracle [Chanoch Lanar 26; see 343/3] which is approximately the age of six or seven. [Kaf Hachaim 676/26] However some say the age of Chinuch in this regard is nine years old. [Chelkas Yaakov 3/144; Leket Yosher of Terumos Hadeshen 158; See Piskeiy Teshuvos 689 footnote 33] The Mitzvah of Chinuch applies even if one assesses that the child will not be able to focus throughout the entire Megillah reading. [Piskeiy Teshuvos 689/8 footnote 35] However, in such a case, a blessing is not to be recited if one is reading only for the child to fulfill his obligation. [ibid footnote 36]
 Biur Halacha 689 “Es Haketanim” based on Michaber 689/6
 Biur Halacha 689 “Cheresh”
 This possibly applies even according to the ruling of Michaber in 689/2 that a Cheresh cannot read for others, as from the Beis Yosef it is implied that he is nevertheless obligated to read it himself. It certainly applies according to those Rishonim and Poskim that argue on the Michaber and rule a Cheresh can read for others. Nevertheless from the simple wording in the Michaber ibid it is implied that he is completely exempt. [Biur Halacha ibid; See Halacha 3]
 According to the explanation of the Biur Halacha ibid it is implied that the deaf person is to say a blessing prior to the reading. However since the simple understanding of the Michaber implies that he is completely exempt therefore one may not say blessing as Safek Brachos Lihakel. Vetzaruch Iyun!
 See Piskeiy Teshuvos 589/3 regarding Shofar; Likkutei Sichos 21/497; Shulchan Menachem 3/105
 M”B 589/4 regarding Shofar; Likkutei Sichos 21/497; Shulchan Menachem 3/105
 As he is obligated in the Mitzvah. However see Ketzei Hamateh 589/1 that this only applies if he is able to hear somewhat without the aids, otherwise he cannot blow for others.
 Rebbe in Shulchan Menachem 3/319 [regarding Megillah] and 3/105 [regarding Shofar]; Daas Torah 689 based on Halachos Ketanos; Mahraiy Engel Brachos 25; Minchas Elazar 2/67; Minchas Shlomo 1/9; Eretz Tzvi 1/23; Minchas Yitzchak 1/37 and 3/38; Yechaveh Daas 2/68; 3/54; Mishneh Halachos 4/85; Kinyan Torah 1/75; Igros Moshe 2/108; 4/126; Sheivet Halevi 5/84; Piskeiy Teshuvos 689/3; Shearim Hametzuyanim 129/25; Piskeiy Teshuvos 589/3 regarding Shofar
The reason: As a microphone is not the actual sound of the person but an electrical generated sound and hence one does not fulfill his obligation of Shofar with hearing an electric sound imitation.
Other Poskim: Some Poskim are lenient are permit hearing the Megillah through a microphone and the like. [Minchas Elazar 2/72; Mikraei Kodesh 11; Tzitz Eliezer 8/11] The Rebbe ibid negates their opinion stating they were unaware of the mechanics of the electric device as is evident from the descriptions they gave in their own response.
 Minchas Shlomo 9; Piskeiy Teshuvos 689/3
The reason: As a hearing aid has the same status as a microphone which is invalid for hearing Megillah, as explained in Halacha 4. However possibly he is still obligated to read the Megillah, as many Poskim rule a deaf person is obligated to read the Megillah, as one is not required to hear his own voice upon hearing the reading. Therefore he is to read the Megillah to himself, without a blessing.
 See Piskeiy Teshuvos 589/3 regarding Shofar; Shraga Hameir 7/46; Likkutei Sichos 21/497; Shulchan Menachem 3/105
 The reason: This ruling is recorded in Piskeiy Teshuvos ibid regarding hearing Shofar, as one is not Yotzei hearing Shofar if the sound is mixed with an echo [588/2], and the electric hearing aid causes an echo and hence must be removed, even if one is also able to hear without it. However regarding hearing Megillah there is no echo prohibition recorded, and hence seemingly it should be permitted for one to leave his hearing aid on. [So rules Piskeiy Teshuvos 689 footnote 24 regarding reading the Megillah for others] However in truth it would be forbidden to leave the hearing aid on for Megillah hearing if the person is unable to ascertain the difference between the voice of the reader and the electric voice that is regenerated, as if one pays attention to the electric sound and not the sound of the reader he does not fulfill his obligation. [See M”A 690/6 that for this reason one should not read aloud from a Chumash; Levushei Serud on M”A 690/15 and Nimukeiy Orach Chaim 690 that if one did not pay attention to the words he is not Yotzei; Regarding Shofar see: 590/14; 587/1; 592/8]
 Yifei Laleiv 3/1 brought in Kaf Hachaim 689/1; See Chapter 11 regarding the different opinions in whether Achashveirosh ruled the entire world.