.... How loud does it take to burst an ear? ..... How LOUD can Kyouko scream?
It takes an upward of 165 decibels to rupture eardrums, but the closer you are to the source the easier it is to do. And Kyouko, being a yamabiko she is was only (technically) limited by her 'ability to reflect sound waves'; if you subject her to voice of Big Bang happening she probably could copy that.