I broke my ankle in my martial arts class back in '06. We were doing running drills, and I should've been on the balls of my feet, but I have bad posture and had my weight on my heels. My foot locked into the mat, and my body rotated, but my foot didn't, and something in between had to give. People said they heard it loud and clear, from clear across the auditorium/gymnasium space where we practice. I don't remember hearing a sound but I do remember feeling the snapping sensation running through my entire body, like movie sound effects.
It didn't really hurt, but was shocking, and when I hit the mat I instinctively lifted my leg off the ground rather than trying to stand on it. It looked super messed up - my foot was pointed in totally the wrong direction. I didn't think it was broken though, because it didn't hurt. I was hoping to hell that it was just dislocated, but my sensei refused to try to reduce it (quite smartly - one of my nerves was resting against a bone shard and I probably would've lost a lot of functionality in my foot.)
It was a really funny experience. I was very calm the whole time. Most of the students seemed to be freaking out; before my teacher got called over, I had to tell people what to do, which was strange. The stupid ambulance and emergency room people did a really bad job with me, and had me supporting my leg with my own strength for hours... it ended up really screwing up my hips and lower back, in some respects even worse than what happened to my ankle.
~Joe