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The laws involved in hearing the reading:
A valid reader: The reader of the Megillah must be a Halachicly valid reader, as explained in the next Halacha.
Intention to fulfill the obligation: Both the reader and the listener must intend to fulfill their obligation with the reading. The reader must have in mind to fulfill the obligation of the listener and the listener must have in mind to fulfill his obligation with his listening. One can assume that the Chazan has in mind to fulfill the Mitzvah for all the listeners, including listeners that are behind the Shul and cannot be seen. [One can likewise assume that one who came to Shul for the reading did so with intent to fulfill the Mitzvah. However initially one must explicitly intend to fulfill his obligation prior to the reading. It suffices for the reader and the listener to have this in mind before the start of the reading, for the entire duration of the reading, and he is no longer required to consciously think of it during the reading.]
Hearing every word: The listeners must hear every word from the reader, as will be explained in Halacha 15. If one heard every word he fulfills his obligation even if he did not have intention by every word. [However some Poskim rule this only refers to one who did not pay attention to the meaning of the words, however if one was thinking of other matters while hearing the words then it is considered as if he did not hear those words.]
The blessings: Regarding if the listeners are to say the blessings themselves-See Halacha 10!
Must a person reading the Megillah to himself say it loud enough for him to hear?
Some Poskim rule one must hear himself when reading the Megillah in order to fulfill his obligation. Most Poskim however rule that Bedieved it is valid even if one did not hear the words that he read.
 Michaber 690/14; M”B 689/4
 If one did not have in mind to fulfill his obligation then he must hear the reading a second time [M”B 690/47] however without a blessing. [Admur 213/4]
 Michaber ibid
 M”B 690/49; See M”B 60/10; Admur 589/8-9; Hence they fulfill their obligation even if they did not explicitly have this in mind. However those that are passing by the Shul, and happen to stop to listen to the reading, do not fulfill their obligation unless they explicitly have this in mind. [M”B ibid in name of Peri Chadash] If however they are in doubt in whether they had this in mind then we assume they did have it in mind being that they stopped to listen. [See Admur 589/8]
 M”B 60/10
 M”B 690/48; Kaf Hachaim 690/81
 M”A 690/15; Kaf Hachaim 690/82
 Levushei Serud on M”A ibid; see also Nemukei Orach Chaim 690
 Tzemach Tzedek Even Haezer 323; Shaar Hatziyon 689/7 based on explanation of Beis Yosef for why a deaf person cannot read the Megillah,+ brought in M”B 689/5
 Kaf Hachaim 689/8 based on Bach; M”A 689/3; P”M 689 A”A 3; Peri Chadash; Shaagas Aryeh 7; Gr”a; Erech Hashulchan 689/2 in name of Rashba, Ran, Rashbatz, Meiri, Riaz; Abudarahm brought in Kaf Hachaim 692/2