What is the proof for the general rule that the woman’s voice is not ^awra (i.e. a matter to be concealed)?

In addressing the wives of the Prophet, Suratul-Ahzab, Ayah 32 means, “And say good words.” The hadith related by Al-Hakim in his book Al-Mustadrak mentions that “Al-Ahnaf Ibn Qays said, ‘I heard the hadith from the mouths of Abu Bakr, ^Umar, ^Uthman and ^Aliyy, and I did not hear it as beautiful as I heard it from the mouth of ^A’ishah.’”