On level 12 on the hero side in story mode I'm trying to find these keys, but there are metal boxes with Dr. Eggman's picture on them that I can't get through. I've tried whistling, digging, and throwing stuff at them. Do you know how to break them?
In the beginning of the level(Death Chamber), hit the hourglass that is on the platform you're facing. Head through the door that opens in front of you, you'll see regular wooden crates to the left of you. Break them and there will be an upgrade. Get it, and now you'll be able to break the steel containers.

~ SA Tails