halacha - Are Orthodox Jews opposed to women singing in public, particularly if the woman is singing to a fussy baby in an attempt to soothe it?

My sister lives in Williamsburg, Brooklyn, New York City, and the neighborhood has a large population of Orthodox Jews and, I think, Hassidim (I don't really understand the difference, but that is a subject for another question). She has two kids, both young, and when they were babies, she would sometimes sing to them as she pushed the stroller or rode the bus or subway.

She frequently noticed that when she sang, Orthodox Jewish men would give her nasty looks. She didn't care very much, because it was a public place and there aren't any laws against singing, but I am curious as to why she got the "stink eye".

I heard some vague rumors about Orthodox Judaism and women singing, but I haven't been able to find any conclusive evidence one way or the other. Poking around on this site, I came across this quote:

The Sedei Hemed was relying on the Divrei Heifetz (113b), who "stated that as long as a woman is not singing sensual love songs, and as long as a man does not intend to derive pleasure from her voice, there is no prohibition, such as if she is... singing a lullaby to a baby, or is wailing at a funeral." [4] Rabbi Weinberg also cited the Sefer ha-Eshkol (Hilkhot Tefillah sec. 4 or 7), that listening to a woman sing is prohibited only where there is sexual pleasure. Rabbi Weinberg reasoned that if the Sedei Hemed could permit funeral dirges due to their lacking sexual pleasure, then he could permit Shabbat zemirot on the same grounds. It is obvious that we today can likewise permit by the same logic any song which does not lead to sexual thoughts. Thus, this interpretation that kol ishah is like etzba ketana, i.e. permitted where sexual pleasure is absent, is not only apparent from the simple meaning of Rambam's words, but is also endorsed by Rabbi Yehiel Weinberg.
- Source, cited in this answer

My sister's singing obviously falls under the category of singing a lullaby to a baby, so it would seem that some people agree that there isn't a problem with it.

Is there a reason why Orthodox Jewish men seemed to take offense at my sister singing?

Answer

The Parameters of Kol Isha by Rabbi Howard Jachter addresses this. See the full article for the details, but here are some excerpts:

The Gemara (Berachot 24a) states, “The voice of a woman is Ervah, as the Pasuk [in Shir Hashirim 2:14] states ‘let me hear your voice because your voice is pleasant and appearance attractive.’” Rashi explains that the Pasuk in Shir Hashirim indicates that a woman’s voice is attractive to a man, and is thus prohibited to him. [...]

Both Rav Ovadia Yosef (ibid) and Rav Yehuda Henkin (Teshuvot Bnei Banim 3:127) reject the claim that this prohibition does not apply today since men nowadays are accustomed to hear a woman’s voice. These authorities explain that since the Gemara and Shulchan Aruch codify this prohibition, we do not enjoy the right to abolish it. The Gemara and its commentaries do not even hint at a possibility that this prohibition might not apply if men become habituated to hearing a woman’s voice. Thus, all recognized Poskim agree that the prohibition of Kol Isha applies today.