The "songs" that Kino has are his feelings. The song represent what he feels at the moment. That's why they always change because he has different emotions each time. Like when the part of the scorpion happened he heard an evil song because he felt like the scorpion was evil.
I can't really relate with the song thing that Kino has because when I feel anger I just feel anger I don't really hear "songs" when I'm feeling any emotion.