Judaica Press commentary:
songstresses - While hearing a female singing voice is considered an impropriety (Talmud Berachot 24a), Targum Jonathan Ben Uziel seems to indicate that these female entertainers did not actually sing but only played musical instruments. See Talmud Succah 51a where it is indicated that the term "mishorer" applies not only to one who sings but also to one who plays a musical instrument. D. Cohen, in his "Notes on II Samuel" (Hadorom, Nisan 5732), suggests that perhaps these songstresses were pre-menstruate, when their voices were not yet considered sexually stimulating. See also Responsa of Seridei Eish, v. II. [I believe that R. Weinberg's responsa permitted mixed singing due to the inability of distinguishing male & female voices, and felt that leniencies should be made as a response to inroads made by the Reform movement. - FH]For additional biblical references to women singing in the presence of men see here. Not surprisingly, exegetes universally rely on extreme stretch & kvetch apologetics rather than admit to the obvious - that the prohibition of kol isha was a much later rabbinic decree rather than a precept based on biblical sources.