[Popman]

Since breaking the speed of sound depends on the air density one is passing though, at what point will the break the sound barrier?

Sea level ???

[11Bush]

Sea level ???

Hopefully before ground level.

[FredZarguna]

By the time he reaches the lower atmosphere, around a few thousand feet, he will not be travelling much more than 300 MPH -- if his chute does not open. He will probably need a breaking chute in order to get down to 110 MPH for the main chute not to tear parts of his body off, or shred.

He must break the sound barrier while his terminal velocity is over about 670 mph (at higher altitude the speed of sound decreases.) This suggests he must break the speed of sound somewhere near Kittinger's record, which would be somewhere around 90,000 feet.