I was in a café today and I could hear a woman singing. No, there was not some sort of open "mic" thing. Nor was she was a scheduled performer. Yes, I was wearing headphones. No, I do not believe that she has any serious mental difficulties.
What the heck?
I don't think there are any etiquette rules about this. Because, seriously, who does that? You are not in your house!
I like the Cure. I probably don't like the Cure as re-imagined by you. I almost certainly don't like the Cure as re-imagined by you cutting in on Jonathan Agnew telling me about England's latest batting collapse.
It is a public place! You owe people the courtesy of letting them pretend you are not there!