Boomboxes are obnoxious. As is any other way of playing music in public places that disturbs others. That’s why we have headphones – so that we can listen in public, privately, and without bothering anyone else.
Headphones are great if you want to listen alone but not so good if you want to include others in what you’re listening to. It’s a problem we haven’t really solved yet. You don’t want a solution that includes wires. Wires restrict movement. Two people walking down the street connected by their headphones. Wires make it awkward and weird.
It needs to be a wireless solution. The problem is easily solvable with today’s technology. You just need an app. As I’m not a programmer there are likely obstacles I’ve not thought of. Though I suggest that these problems are not with the possibilities in the programming language but with what is permissible by the gatekeepers. Things such as interoperability  between competing mobile operating systems (iOS, Android, Windows and whatever else). Or restrictions put in place on developers by licences.Â
The app would simply need to allow you to broadcast the audio you’re playing (via bluetooth, wi-fi or other), to a localised area (twenty metres or so) so that people you approve would be able to listen in (once they are within that localised area). You should be able to play playlists from various music services by connecting your account. Same with podcasts and any audio you have on your device.
Another feature of the app would be the ability to speak to those listening to the broadcast without removing your headphones. This would be great for people running together who want to listen to music and talk at the same time. It could even be used by a coach to pass instructions to those in the group. Using this feature could be independent from the music so that everyone can choose their own music to listen to but still be able to hear and speak to the rest of the group.
No doubt there are issues that would come up in development but I have no doubt that they would be solvable.