ARE YOU PEOPLE DAMN STUPID!?
The question is not "which was the first vehicle to break the sound barrier". It's certainly not "which was the first land-based vehicle to break the sound barrier."
The answer, as SO MANY PEOPLE before me have stated, is the WHIP! The crack is caused by a sonic boom when the tip breaks the sound barrier. The whip broke the sound barrier before the bullet, before the radio or the telephone, before the jetplane and CERTAINLY before the DAMN ROCKET CAR, YOU FRIGGIN' IDIOTS!
Also, rac_fleming, no thrown object has ever broken the sound barrier. That would mean it was moving at the speed of sound. The speed of sound is in excess of 1000 ft. per SECOND. And even if a rock had been thrown at the speed of sound, it wouldn't count, because man did not invent it.
USE YOUR DAMN HEAD!