So now there is two obnoxious people blaring sound? If you didn't have the courage to speak up, how are you going to have the courage to disrupt them and others?

The fact that this occurred in Bombay is important context. In India, the culture amongst older people is to have a clear sense of where you fit in the hierarchy. You might be verbally abusive to those who you consider below you, but you will remain silent and deferent to those who are considered economically/socially superior. This manifests as a certain class of people who have never been called out on any of their obnoxious behavior, because their economic/social status has shielded them from criticism for their entire lives. Meanwhile a majority of society is perfectly accustomed to being verbally abused, to the point where someone like me saying "please" and "thank you" makes it clear that I am of the Indian diaspora.

By the way, I've noticed that the younger crowd in India leans much more toward egalitarianism and tends to reject bizarre social constructs like caste. The fact that a young guy also thought of this solution speaks to their ingenuity as well.

Think it through just a tiny bit more. It’s more socially acceptable to be angry back at someone who is confronting you directly than someone who may or may not be making an example of you but in a passive way. Therefore it’s less likely the other individual will confront you back, or perhaps more importantly it would make them look more unreasonable for doing so.

Social pressure is a real thing and it affects both behaviour and outcomes, it’d be silly to ignore that.

You don't have to figure out what to say back to the person. It is hearing their own self that makes them want to STFU. Apparently hearing their voice is just as annoying to them as it is to us?

Does it really take "courage" to speak up in cases like this? If anything, it's just as insulting to point out to an adult that playing loud audio in a crowded public place is inappropriate, as if they didn't know that!