"A woman's singing is impurity." because the song can stir something inside him then his thoughts begin to race, and that might lead him to thoughts of sin- and other thoughts that he is not allowed to think about but if he hears a woman's voice who he doesn't know- he doesn't know who this lady is who is singing- the letter of the law states that he is permitted to hear her sing or even if he does know her but she has died, she's no longer alive, he may listen to her voice even though he knows who she is-if she's no longer alive there's no fear that he may have thoughts of sin but if she's alive and he hears her voice, it is something else and regarding this it says: 