1) Gravity scales as 1/R^2 where R is the distance from the centre of the Earth.

2) Air density scales as e^-h where h is the scale height of the atmosphere (a few km)

the equatorial radius of the Earth is 6371 km. at 5 km altitude the air pressure is about 1/3 of what it is at sea level but gravity is only 2/10 of 1% lower.

Don't be confused by any discussion of getting "beyond Earth's gravity field with a rocket"

The Moon is 400,000 km overhead and it is still bound by Earth's gravity.

And yes, the process of "breaking" the sound barrier would be accompanied by buffetting. The trick is to do it at a high enough altitude that the buffets are feeble...