Q. Why does Jewish law object to women singing in front of men?
I will come back to the b in front of ishah, but let me first address your main question.
It is not only Judaism that has a problem with female voices. A French writer, discussing the opera and its music, says, “She who sings must die”. Female characters in many operas do die by the time the performance ends. The idea seems to be that the more rapturous a woman singer becomes the more she enters another realm.
Men both want and don’t want to hear the female voice, and several religions, not just Judaism, seek to control it.
The halachic objection applies especially during religious worship and especially when the singer is both seen – the b in front of ishah indicates “with the (sight of) the woman” – and heard.
Though many translations render ervah as “lustful” or “impure”, deriving it from ur, “to be bare”, I recommend a translation which links the word with a verb that means to awaken or arouse.
There is a form of this verb in L’chah Dodi when we say Hit’orari, from a root which is also spelled ayin-vav-resh, to wake. The passage calls upon Zion to awaken at the coming of the Messiah.