What should I do when I hear a sound resembling music, and I don't know whether it is music or not? And should I admonish someone who does that? And how can I determine if a sound is music or not?

Any sounds emanating from instruments that are melodious and rhythmic fall under the ruling of forbidden musical instruments. As for natural human voices not enhanced by instruments, they are not considered musical instruments, even if they are melodious, and thus, one should not condemn those who listen to them.