Megillah 21b: The Targum should not be read publicly by two people in unison (Rashi: because two voices cannot be heard) but Hallel and the Megillah can be read even by ten people in unison. Why? Since it is beloved to the listeners, they will pay attention and hear.
מגילה כא ע”ב: תנו רבנן: בתורה אחד קורא ואחד מתרגם, ובלבד שלא יהא אחד קורא ושנים מתרגמין. (פירש רש”י וטעמא משום דתרי קלי לא מישתמעי.) ובנביא אחד קורא ושנים מתרגמין, ובלבד שלא יהו שנים קורין ושנים מתרגמין. ובהלל ובמגילה ־ אפילו עשרה קורין ועשרה מתרגמין. מאי טעמא? כיון דחביבה ־ יהבי דעתייהו ושמעי.
Rabbi Yaakov Yitzchok Ruderman, in a conversation with some Torah educators, graduates of Yeshivas Rabbeinu Yisroel Meir Hakohein, noted the special stress Slabodka Yeshiva put on interpersonal relations, and told the following story. (It was apparent in his tone that he was unsure whether to tell the story to his interviewers – but he did.) In the 5690’s (1930’s), he was once invited to spend a Shabbos at the home of the Rav of Elizabeth, New Jersey. Before the arrival of Shabbos, the rav took him aside for a confidential exchange of words. He told his guest that he had several daughters eating at the Shabbos table who enjoyed Shabbos by singing the zemiros along with him. (At this point in the narrative, R’ Ruderman interjected that there were no Bais Yaakov schools in America at the time – indicating that Bais Yaakov graduates would know better than to do this.) The rav told him that R’ Boruch Ber Leibowitz had been a guest at his home sometime earlier, and when the girls began singing, he stood up and ran out of the room – perturbing the Shabbos for the girls and humiliating their father. The host then asked whether R’ Ruderman would do the same. The guest replied that with his Slabodka background, he would not destroy the family’s Shabbos spirit or embarrass his host; he would remain sitting and not listen to the girls. “My frumkeit does not have to hurt others,” he concluded.
[In Kol Isha there are two separate prohibitions: 1) hearing it, and 2) saying holy words and prayers while hearing it. For a father, the first is not a problem – a father is allowed to hear his daughters sing. Only the second problem exists. But a guest has to contend with both problems.
The Otzar Haposkim Even Ho’ezer 21 brings that the Chasan Sofer permitted listening to a man and woman sing together, because of the principle of our Gemara that “two voices cannot be heard.” In response to the argument that a woman’s voice is חביבה because it is attractive, he argues that one only becomes attracted to her after hearing her sing, but since he cannot hear her sing due to the principle that “two voices cannot be heard,” the problem doesn’t begin.
The Otzar Haposkim then brings the Be’er Yehuda, who argues based on Rashi in Rosh Hashanah 27a that the reason why “two voices cannot be heard” is because they do not speak exactly in unison; one is always a little ahead of the other. But when they are singing, everyone sings together and they can be heard.
Moreover, it would seem that the Gemara uses this principle as a chumra, not a kula. The Gemara is not saying that when two people read from the Torah, the listeners will never hear a single word. They will probably catch a few words here and there, but they cannot fulfill their obligation that way. But when a woman is singing with a man, and you are looking to be lenient and allow a guest to listen based on “two voices cannot be heard” – this line of reasoning does not work, since at least some of the time he will hear the woman’s voice.
In light of the above, we could suggest a middle ground between these two opinions: for a father to sing with his daughters would be allowed, since we could say ממה נפשך: either way there is no problem. At the times when the daughters are singing at the exact same time as him, he does not hear them because “two voices cannot be heard” and therefore there is nothing wrong with him saying the holy words of Zemiros. (This logic would not apply to one’s wife singing, since her voice is חביבה to him.) And at the times when they are singing separately from him and he does hear them, there is no problem because he is their father and is permitted to listen to them – since he is not saying holy words at that moment. However, for a guest, no such ממה נפשך argument can be made.
Regarding the apparent machlokes between R’ Boruch Ber and Rav Ruderman on what to do if already stuck in the situation, we can assume that Rav Ruderman relied on the principle of אי אפשר ולא קמכוין (פסחים כה ע”ב) – that if one is walking on his way and is being exposed to a forbidden pleasure, if he has no way to reach his goal without passing this place, and he has no intention to derive pleasure, it is allowed. See the Mishnah Berurah 75:17 quoting the Chayei Adam:
ומ״מ אם הוא בדרך בין העכו״ם או בעיר והוא אנוס שא״א לו למחות כיון דלא מצינו דמקרי ערוה מדאורייתא מותר לקרות ולברך דאל״כ כיון שאנו שרויין בין העכו״ם נתבטל מתורה ותפלה וע״ז נאמר עת לעשות לד׳ הפרו תורתך אך יתאמץ לבו לכוין להקדושה שהוא עוסק ולא יתן לבו לקול הזמר.
The Mishnah Berurah permits even saying holy words while listening to a woman singing because of עת לעשות. The implication is that for being on the road among such people, without saying holy words, we don’t need any special heterim; that is the regular rule of אי אפשר ולא קמכוין. In Rav Ruderman’s case, although it was possible to leave the room or, having been warned, find somewhere else to spend Shabbos, he held that that was considered אי אפשר since it would be impossible to do so without embarrassing another person.
R’ Boruch Ber, on the other hand, probably held that although personally, he could have been lenient, he hoped that his actions would teach the family that it was unacceptable for girls to sing in front of a stranger. Rav Ruderman, however, realized in retrospect that they had not learned from R’ Boruch Ber and would probably not learn from him either.]
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