What should I do if I hear a person's phone ringing with a musical tone in the street, and is it obligatory for me to advise him despite my fear of being harmed?

Musical ringtones for mobile phones are a reprehensible act (munkar). If you are able to object to the person using them and advise them to change it to a ringtone that does not contain music, then you have fulfilled what you are commanded to do by the Sharia, based on the Prophet’s (peace be upon him) saying: “Whoever among you sees an evil, let him change it with his hand; if he is unable, then with his tongue; if he is unable, then with his heart—and that is the weakest of faith.” As for fearing harm, if it is based on a real situation, then you are permitted to refrain from advising. But if it is merely illusions, then no attention should be paid to them.