Nowadays it wouldn't appear particularly exotic (portable speakers are ubiquitous) and people playing loud music (from speakers or phones) seem to do this for themselves without caring about the surrounding people, that's just highly egotistical and self-centered. It's usually not music I personally like too.
That's not fun, it's just annoying. There's already too much noise in our cities.
But if you do it right, playing (singular) music for others, reading the room and all, I'm sure it can still work.
Not always a bike. Sometimes just a trolley or a handcart or a custom jerry-rigged contraption.
Okay, in all seriousness: boom boxes were a thing in the 1970s and early 1980s. There are still people who use the latest generation of them, but thankfully not nearly as many. These days people cranking music in a public place are generally doing it as part of a group that all want to hear the same thing. (and in back then the group wanting to hear the same music was the common case)
Second is probably motorcycles blasting it over the engine noise.