Gemara

Rav Nachman: Would you like to send regards to my wife?

Rav Yehudah: (She will respond, and) one may not hear a woman's voice.

Berachos 24a (Shmuel): A woman's voice is Ervah -- "Ki Kolech Arev".

Rishonim

Rambam (Hilchos Isurei Bi'ah 21:2): One may not gesture or wink at an Ervah... one who does is lashed mid'Rabanan. One may not even hear her voice.

Rosh (Berachos 3:37): A woman's voice is Ervah. This refers to hearing it, not to Kri'as Shema. -- "Ki Kolech Arev".

Ma'adanei Yom Tov (80): Chachamim were lenient about Kri'as Shema, lest one miss saying it if.

R. Yonah (Berachos 17a DH Ervah): One may not say Shema when a woman is singing, for her voice is Ervah. It is permitted when she talks normally, or even when she sings, if he can concentrate on Tefilah and not hear or think about her. He should not interrupt Kri'as Shema (in such a case).

Mordechai (Berachos 80): Rav Hai Gaon, and Bahag and R. Chananel, say that a woman's voice is Ervah regarding Kri'as Shema. The Re'em forbids saying any Devar shebi'Kedushah when hearing a woman's singing voice. We live among Nochrim; "Es La'asos la'Shem Heferu Torasecha" (we must transgress, for the sake of Torah). Therefore, we are not careful not to learn while hearing Nochriyos singing.

Rashba (Berachos 24a DH v'HA d'Omar): We are not concerned for one's wife's speaking voice. He is used to it, and it does not distract him. One may not hear another woman's voice, even of speech (not song). This is why Rav Yehudah did not greet Rav Nachman's wife. It seems that this is only regarding She'elas Shalom or returning Shalom, like the case there, for it leads to fondness. The Rif did not mention an exposed Tefach, thigh, hair or voice. The Ra'avad says that the Rif holds that since we said that buttocks are not Ervah, all the more so these. The Ra'avad himself says that we are not concerned for buttocks because they do not distract a man.

Beis Yosef (OC 75 DH Kosav ha'Rashba): The Rambam does not mention these regarding Kri'as Shema. It seems that he holds that they are forbidden to hear or see; the Isur was not said regarding Shema. L'Halachah, we hold like the Rambam, but l'Chatchilah one should be careful at the time of Shema.

Poskim

Shulchan Aruch (OC 75:3): One should be careful not to hear a woman's singing voice at the time of Kri'as Shema.

Beis Yosef (DH Tefach): The Ra'avad says that a woman's voice not of song is not Ervah.

Kaf ha'Chayim (22): One may hear speech even of a married woman, even at the time of Shema.

Beis Yosef (DH v'Chosav ha'Rosh): The Rosh says that Shmuel does not refer to Kri'as Shema. This is why the Tur omitted this.

Mishnah Berurah (16): If he transgressed this, he should read Shema again without the Berachos.

Kaf ha'Chayim (27): Hair and voice are not Ervah mid'Oraisa, so b'Di'eved one was Yotzei, and it helps to close one's eyes.

Mishnah Berurah (17): We are concerned even for a single woman's voice. It is permitted not at the time of Shema, but he must not intend to enjoy this, lest he come to Hirhur (lustful thoughts). One may never hear the singing voice of a married woman or any Ervah, even a single woman who is Nidah. Nowadays we assume that single women are Nidah from the age that it is normal to see Dam Nidah (adulthood). A single Nochris is considered Ervah. A Yisrael or Kohen may not hear her sing. However, if one is away among Nochrim, or is in the city and cannot protest, since we do not find that this is called Ervah mid'Oraisa, one may read Shema or bless at the time. If not, we would be Batel from Torah and Tefilah; "Es La'asos la'Shem." One must focus on the Divrei Kedushah, and not pay attention to the singing.

Kaf ha'Chayim (23): Every woman, even single, must be careful that no man hear her sing.

Kaf ha'Chayim (24): Chachamim did not distinguish whether she is old or young, beautiful or repulsive.

Rema: This applies even to one's wife. A voice he is used to is not Ervah.

Mishnah Berurah (18): Since he is used to it, he will not come to Hirhur. This is even if she is married. One may not intend to enjoy her speech. One may not even look at her clothing for enjoyment!

Birkei Yosef (271:1): The Shulchan Aruch says that a woman may be Motzi a man in Kidush, but some say that for Kri'as ha'Megilah she may not. Here he brought only the opinion that he follows. Ba'al ha'Divros says that she may not read the Megilah for men due to Kol Ishah, but she may bless on Ner Chanukah, for men need not be there. If so, also Kidush is a problem!

Ha'aros on Birkei Yosef (2): Some say that women are not commanded to read the Megilah, only to hear it, but the Shulchan Aruch does not rule like this. Women may not read the Torah b'Tzibur due to Kavod Tzibur (Megilah 23a). Kol Ishah was not an issue! Nichmad l'Mar'eh (on the Yerushalmi) says that we are not concerned b'Tzibur, for the fear of the Shechinah is on men.

Shulchan Aruch (EH 21:1): One may not listen to the voice of an Ervah. One who intends to do so is lashed mid'Rabanan. This applies also to a woman forbidden by a Lav.

Beis Yosef (DH u'Mah she'Chosav ha'Miskaven): Even though the Rambam mentioned lashes regarding other matters (gesturing...), the Tur understands that it applies also to hearing her voice, which he wrote after them.

Beis Shmuel (4): One may hear the voice of his wife or a single woman, except for the time of prayer.

Prishah (2): The Maharshal permits hearing the voice of one's wife or a single woman. Any woman's voice at the time of prayer. Some explain that we forbid a singing voice, for this is Ervah, but permit speech. This is difficult. Why should we forbid his wife's voice not at the time of prayer?!

Pischei Teshuvah (YD 195:10): Perhaps one may not hear his wife sing when she is Nidah. Shabbos 13a connotes that whatever is Asur with an Eshes Ish is Asur with Ishto Nidah, except for seclusion. This requires investigation.