The Issur of Kol Isha-Listening to her voice when not Davening/learning:
It is forbidden for a man to [ever] listen [i.e. pay attention] to the [singing] voice of a woman who is an Erva [such as a married woman, or a Nida, even not during times of prayer and learning].
May a man hear a woman singing if he does not know who she is?
May a man hear many women singing simultaneously?
Must a man leave or not enter an area in which a woman is singing?
One is not required to leave, or not enter an area that contains a woman singing. One may thus enter a bus or taxi or store even though the radio is on with a woman singing. He is however required to remove his mind from the voice and pay no attention to it. If one sees that despite his efforts it is leading him to immoral thoughts, he is to leave the area.
Q&A on women singing
May a woman sing if there are men present who can hear her voice?
May girls/women sing in private if their voices can be heard to the outside?
At an event, such as a Farbrengen, is it permitted for women to sing even though their voices can be heard to the outside?
As stated above, it is forbidden for men to listen to a womenâ€™s voice of singing even if there are many women singing simultaneously, and even if he does not see the woman. It is therefore proper for women and girls to avoid singing in areas that men can hear their voices. This applies even towards girls in school and the like, when singing for spiritual purposes. However, some Poskim rule that one cannot protest those girls who gather in private to sing as part of their spiritual education, such as the singing of Niggunim or Davening. In such a case, the men are obligated to remove their minds from the women’s voices.
 Michaber E.H. 21/1; M”A 75/6 in name of Lechem Chamudos Brachos 3/117; Admur 75/6 as well as the other Poskim in 75 do not deal with whether one may listen to a woman’s voice during other times, and the law is only in reference to Davening/learning.
 Biblical or Rabbinical: According to some Poskim, the singing voice of an Erva is Biblically forbidden for a man to listen to. [Implication of Michaber ibid; Perisha 21/2 on Tur E.H. 21] Other Poskim however rule it is only Rabbinically forbidden. [Nishmas Adam 4/1; Kaf Hachaim 75/27]
 See Admur ibid; Olas Tamid 75/5; Rabbeinu Yona ibid; Kitzur SHU”A 5/16; Kaf Hachaim 75/26; See Beis Yosef 76
 M”A ibid
 Beis Shearim 53; One way of learning Maharam Shick E.H. 53
 The reason: As the Yetzer Hara only has a lust towards that what he sees. [Poskim ibid]
 Peri Hasadeh 3/32; Ohel Moshe 1/32; Birchas Chaim 21; Birchas Shamayim 30; Poskim in Otzer Haposkim 21; Yaskil Avdi 5/12; Chelkas Yaakov 1/30 and 163; Minchas Yitzchak 7/70; Shevet Halevi 3/181; 5/197-3; Az Nidbaru 9/59; Rav SZ”A in Halichos Shlomo 20/12
 The reason: As one is not allowed to hear a woman’s voice even if he does not know her identity, as the rule that the Yetzer Hara only has a lust towards that which he sees was said only regarding future erotic thoughts, however at the time of the hearing of the sound, it certainly has erotic thoughts. [Poskim ibid]
 Shevet Halevi 5/197-4
 See Sdei Chemed Kelalim Mareches Kuf 42; Ohel Yaakov 76 [p. 203] in name of Poskim; See Admur 75/6 based on previous Poskim “Even by the singing voice of a woman, if one is able to concentrate on his prayer to the point that he does not even consciously hear or pay attention to her voice, then it is permitted [to learn and Daven while hearing it].”And certainly it is permitted to be in the area.
 Beir Sheva p. 119, brought in Kaf Hachaim 75/23; Elya Raba 75/5; Shaar Hatziyon 560/25; Piskeiy Teshuvos 75/12
 Although the verse states that Devorah and Barak sang together, nevertheless, this is different being it was done as result of Hashem’s command. [Elya Raba ibid]
 Beir Sheva p. 119, brought in Kaf Hachaim 75/23; Elya Raba 75/5; Shaar Hatziyon 560/25; Shevet Halevi 5/197-4; Halichos Bas Yisrael 6/6; Piskeiy Teshuvos 75/12; see Beis Avi 3/32
 Shevet Halevi ibid
 Beis Avi 3/32; Ishei Yisrael 55/29 in name of Rav Elyashiv, brought in Piskeiy Teshuvos 75 footnote 125; Halichos Bas Yisarel ibid footnote 12 brings in name of Rav SZâ€A that girls may be lenient to sing on a bus with separate compartments if the male bus driver cannot see them.
 The reason: As perhaps singing out of service of G-d does not bring immoral thoughts, as was seen with Miriam and the women who sang Shiras Hayam. [Beis Avi ibid]
 Ishei Yisrael ibid