[tempestn]

Possibly! Certainly if the plane was traveling in a vacuum it would produce no boom, so one would assume that the boom would be reduced as the air became less dense. However my understanding is that most of the advantage from altitude comes from the attenuation (or really spreading out) of sound with the square of distance.

[mark-r]

Since hearing response is logarithmic, the inverse square law probably isn't as helpful as you'd expect.