Ah, but you forget that BOTH Rina AND Yeshu'ah are singing at the same time!
Chazal say that "Ayn shomin shnay kolos ke'achas," which is why it's impossible to pay attention to your wife when you're watching TV. Since the sound is a mixture of both Rina and Yeshu'ah, it's permissible for them to be singing in the presence of the tzaddikim (Sh”t Sridei Esh 2:8).
But why only the tzaddikim? Since "Lo yamish Yeshu'ah mitoch ha'ohel," we know that he was already present when Rina started to sing. The tzaddikim are zrizim, so they are careful to command Yeshu'ah to sing along with Rina. By rov ha'am, we are afraid that they will be less mindfull and neglect to tell Yeshu'ah to sing, thereby leading to an aveirah of kol isha. That is why we restrict the singing to ohalei tzaddikim.