Sometimes, but not too often, in Berlin of all places you encounter someone biking around with a massive speaker attached to their bike blaring (obviously) techno
Not always a bike. Sometimes just a trolley or a handcart or a custom jerry-rigged contraption.
You see this type of thing daily in Brooklyn NY still, but instead of techno it's any genre, primarily hip hop, reggaeton, or afrobeats.
In the US "boom boxes" went out with the 1970s. They need to get with the times.
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)