Regarding your second to last paragraph, Exodus 15:20-21 might actually be taken as evidence that the women specifically went off on their own and avoided singing in front of the men. Though the verse cited by the Talmud (B'rachos 24a) as indirectly implying a prohibition against listening to kol isha is Shir HaShirim 2:14, since it compares a woman's singing voice to her physical appearance.

Hi JMFB! We don't know you that we should value your personal opinions. Please edit in some sources for your claims if you want to be taken seriously.

@Fred I went to Charedi yeshiva in yerushalayim for two years. Do you really believe that a 600,000 women seperated themselves from the men right after the parting and sang out of range. Come on. It's Deoraysa.

@DoubleAA Which part do you need proof of? Am I not supposed to come in with the rudimentary approach that people here know basic halacha? I'll be happy to elucidate further if you like. Let me know which parts confused you.

@JMFB Any claim you made should be sourced. For instance: Kol Isha falls under Tznius. It was designed by rabbis. It was designed for that purpose. It doesn't apply to non-jews. That it only applies while doing spritual things. Etc.

JMFB (cc @fred) Note that your claims are contradictory. The proof from Miriam is only good if Kol Isha is deorayta. Otherwise, it could have been before the enactment. Note further that Miriam is before Mattan Torah and hence she wasn't Jewish.

@JMFB How is that less plausible than Miriam leading 600,000 women in responsive song without going off on their own. And anyway, as I mentioned, the verses in Exodus are relatively explicit that Miriam's responsive singing was only to the women, and that the women went off as a group and joined Miriam.

@DoubleAA I'll fill it in later for you. You really need proof that it falls under Tznius? There's an argument made by a Rabbi in the Gemara that Kol Isha is Deorisa, but that's not the majority opinion. It's clearly not written in the Torah anywhere, not sure how to prove that other then to post the entire torah here and say it's not in there. Only the noahide laws apply to non-jews. Do you really want me to source such basic ideas?

@DoubleAA I'm not claiming the verses in Exodus as the source for the kol isha prohibition. I was just saying that, if anything, those verses could suggest that the Jews were cognizant of kol isha even back then. And I'm not saying there is some separate d'oraysa about kol isha; I was just citing the gemara in B'rachos that points to the verse in Shir HaShirim as a source for including kol isha in the erva category.

Source anything you can, please. The OP, for instance, is not Jewish in this case, and he and others may benefit from sourcing things which you might find 'simple' or 'obvious'.

Incidentally you are wrong about Kol Isha being deoraita or not being a machloket in the gemara.

BTW @Fred The other Talmud cites a different verse judaism.stackexchange.com/a/28865/759

@DoubleAA I said there is a Machloket and there is a minority opinion that it is, but that is not an accepted opinion.

@JMFB You said it's in the gemara. This is false. Hence your trustworthiness in my eyes is weakened.

@JMFB I'm not saying we paskin from Shir HaShirim (which is in Kesuvim, by the way). I'm saying that the Talmud uses a verse from there to help define the category of erva, not as the direct source for a lo sa'aseh.

We do not poskin from the naviim, do we?

@fred so what is the direct source then?

@JMFB The direct source for what? That a man is not supposed to intentionally expose himself to erva for the sake of getting pleasure from it?

@DoubleAA Good point about the alternative verse. I had actually upvoted your answer there way back when.

@DoubleAA i stand corrected it's one of the achronim, that doesn't make a person untrustworthy.

@Fred Specifically kol isha

@JMFB Making mistakes in citations makes you less trustworthy. That's just to emphasize why you should cite things and not always rely on your memory. And how little we should trust random internet users' memories.

@JMFB Regarding what I wrote above (a man is not supposed to intentionally expose himself to erva for the sake of getting pleasure from it), it's a machlokes rishonim whether that is a biblical or rabbinic prohibition (Tosafos on Avoda Zara 20b, for example, derives the biblical prohibition from Deut. 23:10). Regarding whether the inclusion of kol isha in the erva category is biblical, that is also a disagreement among authorities, but many maintain it is rabbinic.