As others have stated here, you can hear the sonic boom of planes flying by at a fairly high altitude, ~80 000 ft. At what altitude would the sonic boom no longer be noticable at sea level? To add to that, is there any relatively "simple" method of estimating sound dissipation with altitude?
As an observer, you hear the Sound Pressure Level (SPL), which comes from a source emitting sound at a given Sound Power Level (SWL).
According to this source, the SWL of a supersonic plane may exceed 200 dB. In the same source, a formula for computing SPL a given distance is also shown.
You can find an online calculator for that here.
Dissipation of sound waves over distance depends almost entirely on variety in both composition and density of the material it travels through. This is why vertical propagation of sound is way more restricted than horizontal. So much so in fact, that a sonic boom generated by an aircraft at high altitude clearly heard at sea level at one location, may be completely unnoticeable as the same aircraft passes over a location a hundred miles further down its flight path. The altitude is more or less insignificant, unless it is clearly way too close or clearly way too far away. It's the stuff in between that matters.