Nothing, you hear nothing.
(is my best guess answer to this theoretical question, without going into significant scientific and mathematical calculations)
Get yourself a sheet of paper, and hold it vertically from the top, like this:
Now blow some air in the middle of the paper, and you will see that the paper is bending, more at the bottom, not so much in the middle, and very little at the top:
Now we have established that paper doesn't move when you hold it at one point, we can apply this knowledge to a speaker cone with a diameter of 40 miles (= 64km) with a voice coil at 20 miles high (= 32km)
First lets look at the athmospheric pressure by height:
So we can clearly see that at ground level, we have about 1000mb, at 10km we have about 200mb (one fifth of pressure), and going down rapidly from there up.
This air pressure is basically holding the paper cone of the speaker at the bottom, while there is no air pressure at the top, effecively doing this with the sheet of paper:
Now think about where the voice coil is. It is in the middle of the speaker, moving the cone.
However, as the cone has no air pressure at the top, for most of the cone, the paper cone will bend and only move in the upper regions of the atmosphere. There is no "resistance" to move, so it will move. The paper cone, like the sheet of paper, will bend. The atmospheric pressure at the very bottom will keep the cone from moving at ground level.
As you need movement (and air) to reproduce sound, the energy of the voice coil will be distributed to the upper and largest portion of the cone and not move the cone at ground level, nor in the part of the cone which actually has air to move (as we know now that the air is actually holding the cone still at the bottom).
Conclusion: you hear nothing.
Disclaimer: this answer is not scientific, and does not proof this with mathematical formula's. I think I have spend enough time on this already to write a theoretical answer to a theoretical question which is far from realistic.