Can one be Yotzeh Meggilah on Skype or webcast or through a microphone or telephone is there a difference between each? Does a Hearing Aid disqualify a person from being Yotzeh the Laining?
R' Shlomo Zalman Auerbach1 discusses the entire issue of recreated sounds in Shu"t Minchas Shlomo siman 9. His basic synopsis is that since the sound is being converted from sound waves to electrical signals and then converted back into sound waves, the sound one hears is not the original sound in any form.
Therefore, he concludes that one may not fulfill mitzvos dependent on hearing through any means of electronic reproduction, such as radio, microphone, or hearing aid. He instructs that hearing aids be removed in order to hear shofar or megillah. 2
One may answer Amen to a blessing heard over a microphone (for technical reasons, only if the person is not obligated in that blessing themselves) because the digital reproduction does serve to make the person hearing know that a blessing is being said, like in the Talmudic example of the synagogue in Alexandria, where a flag was raised to signal that a blessing had been made.
1 My personal education was that R' Shlomo Zalman was the most reliable and accepted authority of his contemporaries in the area of halacha relating to technology, and most specifically electricity, as he spent much time becoming proficient in the technological side of technology issues.
2 As an aside, I read in Hanoch Teller's biography of R' Auerbach that he was personally very pained to make this ruling, because he himself had sacrificed greatly to acquire a hearing aid for his own mother. Nonetheless, he felt this was the halacha.
Rav Moshe in IG"M O.C.(2) siman 108 and O.C.(4) siman 126 notes that "some experts" say that the sound produced by a microphone is not the actual person's voice magnified, rather it's a new sound. However, he says it seems to still be mutar, m'ikar hadin since you only hear the microphone as a direct result of the person lainig.
However, since it is not clear, and it is a new thing, it should not be done, even if it will result in a delay, and less people there (which would be a chisaron in b'rov am ahdras melech).
(any mistakes are mine alone, as are parentheses)
A very decisive Psak seems That someone who hears the Megilla via radio, telephone, or microphone does not fulfill his obligation. (Shearim Mezuyanim Bahalacha 141:6) Overall it Seems that it is not Allowed