1. It is forbidden for a man to hear a woman sing. This prohibition is called “Kol B’Isha Ervah.” (Brachos 24a and Shulchan Aruch Even Haezer 21)
2. The Sdei Chemed (Kuf 42) cites an argument amongst the authorities whether this prohibition is Biblical or Rabbinic in nature. The Chochmas Adam (4:1), in his commentary Nishmas Adam, concludes that it is Rabbinic in nature. (See Mishnah Berurah 75:17)
Listening To Multiple Women Sing In Unison-
3. The Beer Sheva (Beer Mayim Chayim 3) writes that this prohibition exists whether it is one woman singing or multiple women singing and whether they are singing secular songs or singing Zemiros on Shabbos. This is also the view of most authorities. However, there were those that wished to rule leniently as shall be explained.
4. The Chasan Sofer (Taharas Yadayim 14) extends a lenient ruling based on the Talmudic rule that “Trei kali Lo Mishtamay,” two voices cannot be heard simultaneously. The Gemara uses this principle to prohibit two people to read from the Torah at the same time. Therefore, argues the Chasan Sofer if multiple women are singing there should not exist a prohibition since men cannot focus on two voices simultaneously.
The Tzitz Eliezer (14:7) felt that the Chasan Sofer was not definitive in his ruling and that it is unclear whether the Chasan Sofer would permit this in normative halacha. However, Rav Yechiel Yaakov Weinberg zt”l (Sridei Eish 2:8) records that Rav Azriel Hildesheimer zt”l and Rav Samson Raphael Hirsch zt”l also ruled leniently for the same reason of the Chasan Sofer.
5. The poskim point out that there are two major issues with the permissive ruling of the Chasan Sofer:
1) The rule that two voices cannot be heard, as used in the Gemara, tells us that one cannot concentrate on the words of the speakers and therefore cannot fulfill his obligation for Krias Hatorah. One does, however, still hear the sound of the speaker. In this case, regarding Kol Isha, the major concern is that hearing a women’s voice is inappropriate and considered an Ervah. Even if he cannot make out the specific words he can definitely hear the women sing and it should still be prohibited.
2) The Gemara in Megilah (21) says that two people are allowed to read the Megilah simultaneously. The reason why the Megilah is different from Krias Hatorah is that, “Since the reading of the Megilah is dear to people, they concentrate and can hear.” The same argument can be extended to our discussion. Since the Yetzer Hara will make the man want to hear the women sing then we assume that two voices are able to be heard at the same time.
6. Harav Yechiel Yaakov Weinberg zt”l was asked regarding a German Jewish organization where the boys and girls sang together. Rav Weinberg quotes the earlier heter of “two voices cannot be heard simultaneously” and writes that he does not find this explanation satisfying. Rav Weinberg instead defends the German Jewish practice by citing the Sdei Chemed who “allows for men and women to sing together.”
[However, no where does the Sdei Chemed discuss men and women singing in unison. Perhaps Rav Weinberg was referring to the Sdei Chemed who quotes the Divrei Cheifetz who asserts that the Kol Isha prohibition does not apply to women singing Zemirot, singing songs to children, and lamentations for the dead. This too is problematic since the Sdei Chemed, himself disagrees with the Divrei Chefetz and rules stringently.]
Rav Weinberg contends that when they are singing Zemiros men do not derive pleasure from the woman’s voice (this is perhaps the rationale for the view of the Divrei Chafetz). However, the poskim point out that this is not necessarily accurate and even if the woman are singing Zemiros men can still derive pleasure from their voices.]
7. In summation the overwhelming majority of poskim [including: the Beer Sheva, the Beer Yehuda on Chareidim, the Steipler Gaon zt”l (cited in Journal Ohel Moshe 1992), the Shevet Halevi (4:197), the Tzitz Eliezer, Badei Hashulchan (Nidah 199:119), Chelek Levi, Kinyan Torah (85), Avnei Yashfei (2:5), Ishei Yisroel (55:32), and Netai Gavriel (Yichud page 348)] maintain that a man may not listen to many women sing in unison, even if they are singing Zemiros.
Different Women And Their Status-
9. One’s wife who is not in a state of nidah– A husband may listen to his wife sing when she is not in a state of nidah. However, while he is davening, reciting Brachos, bentching etc. a husband may not listen to his wife sing, even when she is not a nidah. (Rama O.C. 75:3)
10. One’s wife who is in a state of nidah– The poskim debate whether a husband may listen to his wife sing when she is a nidah. The Chemdas Moshe (58) is lenient. Rav Ovadia Yosef zt”l (Yabia Omer 4 Y.D. 15) writes that although it is praiseworthy to be machmir, those that choose to be lenient are permitted to do so. Rav Elyashiv zt”l also rules leniently. (Umekareiv Biyamin vol. 1 page 163)
However, Harav Moshe Feinstein zt”l (Igros Moshe Y.D. 2:75) rules that one should be machmir and not listen to his wife sing while she is a nidah. This is also the view of the Aruch Hashulchan (75:23), Kitzur Shulchan Aruch (153:10), Avnei Yashfei (2 O.C. 5), and ybc”l Harav Azriel Auerbach shlit”a (Levushei Az).
(All agree that when a husband is praying, bentching etc. one may not listen to his wife sing when she is a nidah.)
12. Single Jewish Girls Age Eleven and Up– One may not listen to any Jewish single girl under any circumstance once they have reached the age of nidah. (Pri Megadim 75 M.Z. 2 and M.B. 17) Harav Moshe Feinstein zt”l writes that one should not listen to any girl once they have reached the age of eleven since it is possible that they have already become a nidah.