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.
 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]