Is a woman's voice considered 'awrah (something that must be concealed)? And what is the evidence for that?

A woman's voice is not 'awrah (something that must be covered or concealed). The proof for this is Allah's saying: "And when you ask [his wives] for something, ask them from behind a partition." (Qur’an 33:53). Allah did not forbid speaking, but rather He forbade softening the speech and making the voice alluring. Allah says: "Then do not be soft in speech [to men], lest he in whose heart is disease should covet, but speak with appropriate speech." (Qur’an 33:32). However, if a woman's voice causes temptation or pleasure for the listener, then it is forbidden to listen to it. And Allah knows best.