We know the sound boom - but is there any effect if a body decreases the speed underneath the sound barrier?
Not really, it's just sound instead of a shock.
You can actually see this happen visually. Shock waves in air cause a visible lensing effect, and in the case of bombs exploding you can see this quite easily, sometimes as a sort of distortion and others as a white shell due to the pressure changes cause the water vapour to turn into drops.
If you watch films of this you'll notice that the shell gets to a certain size and then disappears all of a sudden. This happens because the shock wave strength is a function of the difference in air pressure on either side, and once the bomb is done exploding, the pressure (much more slowly) returns to normal. So you have this sort of "step" in pressure moving outward.
As the shock spreads out from its initial location, the pressure differential is being spread out over an ever-larger surface, so the differential is getting smaller all the time. At some point it drops below the point needed to maintain the shock, and it disappears. Since the drop is a function of r^2, this happens very rapidly, which is why it seems to happen over the entire surface at the same time.
Beyond this point you still have a pressure differential, just one that's too small to create a shock. And what do you call a moving pressure differential in air? Sound!
as an object pushing a shock wave through the air slows down, the shock wave transitions from a cone shape to a flat plane at that point where the speed of the air flowing past it is equal to the speed of sound. when the speed of the object falls further and is below the speed of sound, the shock wave expands into an ordinary sound wave which continues on its own at the speed of sound. A person listening to the approach of that object will then hear it coming and not experience a huge "boom" as it passes him.