Isn't your example of flutter.io - holding up a hand for the music to stop - simply another form of button?
Shouldn't a reactive interface stop the music when it sees you raise the phone to your ear?
Shouldn't a reactive interface stop the music when it sees you raise the phone to your ear?
Still, I find using a hand to stop music a more natural interface than hitting a button or switching to iTunes. It's a start, but definitely not the end goal.