WIY,
I am sorry if I gave the wrong impression. I don’t think that most poskim would hold that it is muttar. This heter is given by the Sridei Eish as a limmud zchus on the minhag of the German kehillos where women would sing zmiros on Shabbos and to matir Jewish youth groups in France (akin to NCSY) to have boys and girls sing Jewish songs together. I don’t think even the Sridei Eish meant it as a general heter for all people at all times.