Sonic boom refers to the explosive sound caused by the shock wave from an object traveling faster than the velocity of sound. Yes, It's actually spoken out as breaking the sound barrier.
Felix jumped from an altitude of 39,044 km (which is 128,097 ft.) and reached a peak speed of 833 mph. Yes, He did produce the Sonic boom. Most likely, we use the term Breaking the sound barrier while considering air-crafts like "Concorde" because they could be easily sensed. But in case of Felix, he produced
"It was Mach 1.24. Our ground recovery teams on four different locations heard the sonic boom," said Clark, a former high-altitude military parachutist and NASA doctor who worked on escape systems for space shuttle astronauts.
That "Mach 1.24" reading is comparable to the shock waves produced by Space shuttles...
But how does this number change as air density decreases?
Velocity variation of Sound: Indeed, Sound varies with Temperature and also with Density. Also, the state of matter (which refers again to density). It travels faster in liquids and even more faster in solids (like 5120 m/s in Iron)
In general, the speed of sound in a gas is given by Laplace correction of Newton's formula. For solids, see Wiki ('cause it's not necessary now...) $$v=\sqrt{\frac{\gamma P}{\rho}}$$
Applying Ideal gas law and using density of air ($\rho=1.293$ $kgm^{-3}$), we could find that the velocity of sound increases by 0.61 per degree celsius rise in temperature in air. Also, the velocity of sound in a gas is inversely proportional to the square root of its density. But, it's independent on Pressure (Don't believe in appearance of the formula). This is because increase in pressure, also increases the density of gas. This could be achieved by using different gas densities (at same volume & pressure).
Also, See the Atmospheric variation of sound's velocity.
The flash seen in this video may not be the glow of initial entry since the camera angle is angled somewhat along the path of the meteor; it is likely the active burning after the meteor has already entered the atmosphere. The mesosphere begins about 50 km above the earth's surface. However, the speed of sound in this region is a bit lower than the speed of sound on earth's surface, ranging between $280\text{m/s}$ and $330 \text{m/s}$. Coupled with the fact that we continue to hear less intense popping sounds for more than two full minutes afterwards suggests that the large boom heard is the shock wave, while the following sounds are actually from earlier events.
Meteors tend to enter the atmosphere at between 20 and 50 km/s, although they may be slower or considerably faster than this. This means that if their journey were straight down through the atmosphere, rather than across it, the trip would take 2 seconds or less. The trajectory of the meteor across the sky illustrates that it is a more glancing impact. The fact that the first sound takes 140 seconds to reach the camera indicates that the meteor is about 45 km from the camera. This is not reflective of the height of the meteor, but the absolute distance of the meteor path from the camera. There are estimates that the breakup altitude is around 20km. The following two minutes of reports reflect a linear distance of about 40 km of travel ($120\text{sec}\times300 \text{m/s}=40 \text{km}$), which probably took the meteor about 2 seconds to cover. So what we hear afterward is like a slow motion recording of the meteors breakup history, slowed down by a factor of somewhere between 40 and 100 times. If we could have heard these breakups in a frame comoving with the meteor, it would have sounded nearly like a single, continuous explosion. The first and loudest boom we hear is likely not from initial entry, but rather the sonic boom from the meteor's closest approach to the camera.
Best Answer
According to the American Meteor Society, the sonic boom of an asteroid or meteor (sometimes referred to as a 'fireball') is due to
And from CalTech's CoolCosmos page
The reason for asteroids causing sonic booms in the lower atmosphere, is according to the article How the Falling Meteor Packed a Sonic Punch (Klotz, 2013) is due to
Looking a bit further in to what a sonic boom (Using a jet as an example) is and how it occurs is illustrated in the following diagram
Image source
So, if a meteor, asteroid is going faster than the speed of sound for particular part of the atmosphere, then a sonic boom will occur. Going back to the American Meteor Society's description of the likely cause of a sonic boom, they stated that if a meteor comes in
then a sonic boom is likely to occur, one of the reasons is that the speed of sound is slower, due to the temperature of the atmosphere at that height and lower. Below is a graph showing the speed of sound plotted against temperature as a function of atmospheric elevation:
Image source.