It is true that this app is more hostile than asking someone to keep it down, but people should beware of either approach, as it's not unusual for the same assholes who are comfortable blasting their audio in public spaces to also be comfortable getting into a fist fight.

I have personally been threatened on multiple occasions because I asked someone to turn down (or turn off) their volume while watching videos on their phone in public.

In one instance, I was in a doctor's office waiting room and a rather large, otherwise normal-looking man (likely in his late fifties) was watching videos at full volume while 4-5 of us were sitting quietly. We were all annoyed by him and exchanging looks, so I politely asked him to mute the video or watch it outside and he stood up and started threatening to fight me in a doctor's office waiting room!

In my anecdotal experience in various tier 2 USA cities (i.e., not NY, SF, LA, etc), Gen-Xers and Boomers seem to be the worst offenders and also surprisingly, the most belligerent when confronted.

If you're going to try either approach (this app, or asking), please do not be surprised if you find yourself in a rapidly escalating confrontation that may quickly result in physical violence.

Sometimes, this calculus is more than worth it, sometimes it's not, but just don't think it can't happen.