1. The poskim discuss whether one may answer Amen to a blessing that is said using a microphone and whether one may fulfill any mitzvos, such as havdalah and megillah, using a microphone or telephone. This debate is extremely relevant at weddings, since in most cases the rabbi uses a microphone when reciting the brachos under the chuppah.
2. Harav Shlomo Zalman Auerbach zt”l (Minchas Shlomo 1:9) writes that one may not fulfill any obligations through the use of microphones, telephones, radios or hearing aids. Both the telephone and the public address system “transform” sound waves in air, e.g., spoken words, into an electrical current within the instrument, and, ultimately, back into sound waves. The sound that people hear was not the actual sound waves created by the speaker. This disconnect, between the speaker and the audience, prevents the listener from fulfilling any mitzvos through this medium.
3. Harav Shlomo Zalman Auerbach zt”l explains that when one hears a blessing over the microphone, from a hallachic perspective, one has not actually heard the blessing because there is a separation between the speaker and the listener. Rather, one is merely aware that a blessing is being recited. This is very similar to the synagogue in Alexandria (see Gemara Succah 51b), where most people did not hear the blessings being recited because of its vast size, but were nevertheless permitted to answer amen when signaled to do so by the waving of a flag. Therefore, concludes Harav Auerbach zt”l, one may only respond Amen to blessings that he is not obligated to hear, as was the case in Alexandria, but one may not respond Amen to blessings that one must hear, such as havdalah. He adds, that it is only permitted to respond amen if one is in the same room as the person who is reciting the blessing. If one hears a blessing over the telephone, one may not respond amen.
4. According to Harav Shlomo Zalman one should not recite the blessings using a microphone. If one did, then the assembly may respond Amen. Indeed, the Beis Din Tzedek of Yerushalayim signed a petition against the usage of microphones under the chuppah.(See Koveitz Ohr Yisroel 13)
5. The Chazon Ish (cited in Minchas Shlomo) questioned the view of Harav Shlomo Zalman Auerbach zt”l. He feels that because the person is creating the sound wave and it is heard immediately, perhaps one can fulfill his mitzvah by listening to the microphone or telephone. Harav Moshe Feinstein zt”l (Igros Moshe O.C. 2:108, O.C. 4:91:4) likewise argued that one may fulfill mitzvos using these devices. He adds that every time someone hears havdalah he does not hear the person’s voice in his ears, rather, the speaker causes sound waves which travel from the speaker’s mouth to the ear of the listener. Since one always fulfills mitzvos by hearing sound waves created by the speaker, it may not make a difference whether one hears the original waves or waves that were temporarily converted into electrical currents. As long as one hears sounds that originated from an adult jewish male (without a time delay), one can fulfill his mitzvos. Harav Moshe concludes that in case of necessity one may fulfill mitzvos through listening to a microphone. A similar view is expressed by the Tzitz Eliezer (8:11). [It should be noted that using these machines for the shofar on Rosh Hashana is far more complicated and not part of this discussion]
6. According to Harav Moshe Feinstein zt”l one may respond amen to blessings recited using a microphone.
7. Harav Ovadia Yosef zt”l (Yechava Daas 4:54) rules that one may not fulfill mitzvos through the use of telephones and microphones. However, if one is standing close enough to the one who is speaking that he would have been able to hear him without the microphone, then he may fulfill his obligation. This is true even if he also hears the sound of the microphone and the sound is louder and more amplified. Those who are sitting far away and would not be able to hear him if not for the microphone, may not fulfill their obligations.
8. The common custom is to use a microphone when reciting the blessings under the chuppah. For normative halacha, a rabbi should be consulted.