One can fulfill the mitzvah of listening to the megillah as long as one hears the voice of the person reciting the megillah on one’s behalf. Most poskim agree, therefore, that listening to a live broadcast of the megillah over the radio or the telephone is unacceptable because you are listening to an electronic transmission of the reader’s voice rather the voice itself. According to Rabbi Ovadia Yosef, a microphone is unacceptable. This is because, according to his understanding, the membrane of the microphone absorbs the human voice and then emits an electronic version of it.
According to Rabbi Moshe Feinstein, however, a microphone is halachically acceptable because the transmission is simultaneously activated by the human voice. Rav Yosef concedes that a microphone may be used to amplify the reader’s voice in a case where the reader’s voice would still be audible without it.