Question: I ran out of grape juice on Shabbos and so don’t have enough for havdala. Can I be yotze by watching someone saying it live online?
Answer: The Mishna (Rosh Hashana 3:7) teaches that one who blew a shofar into a barrel only fulfils their obligation to listen to the shofar if they hear the actual sound rather than an echo. The poskim discuss whether a phone or microphone with (virtually) no delay is equivalent to hearing the actual sound or not.
R’ Chaim Elazar Shapira (Minchas Elazar 2:72) and R’ Zvi Pesach Frank (quoted in Minchas Yitzchak 2:113) allow one to use a microphone for mitzvos derabannan.
R’ Moshe Feinstein (Igros Moshe OC 2:108; 4:91:4) writes, however, that as the electronic sound produced is not the actual human voice, it is not ideal to listen to the megillah through a microphone. Nonetheless, bedieved one fulfils their obligation this way. Likewise, he allowed a woman in hospital to fulfil her obligation to hear havdala over the phone when there was no other choice (See Shevet Halevi 5:84). Similarly, R’ Eliezer Waldenberg (Tzitz Eliezer 8:11) permits one to use a microphone for patients in hospital to be able to listen to the megilla when necessary (See Minchas Yitzchak 2:113).
R’ Shlomo Zalman Auerbach, however, disagrees, writing that one cannot fulfil one’s obligation of havdala or megillah over the microphone or phone, even bedieved. He writes that he does not understand how other poskim could have allowed it, comparing the sound heard to witnessing seeing a crime in the reflection of the mirror which cannot be admitted as evidence in beis din. R’ Yitzchak Yaakov Weisz (Minchas Yitzchak 3:38:16), R’ Ovadia Yosef (Yabia Omer OC 1:19:18; Yechave Daas 3:54) and R’ Moshe Sternbuch (Moadim Uzemanim 6:105) agree, explaining that the electronic sound produced is not the actual voice.
The Shulchan Aruch (OC 296:2) writes that one may use chamar medina, popular beverage for havdala. Thus, one may use tea or coffee for havdala (See Aruch Hashulchan OC 272:14; Igros Moshe OC 2:75; Tzitz Eliezer 8:16; Teshuvos Vehanhagos 4:77).
In conclusion, one does not fulfil their obligation of havdala by watching online or listening over the phone. Rather, one should recite it over a cup of coffee or tea if necessary.