This has been an interesting discussion, and prompted me to do a tiny
bit more research.

It seems to me that if there's any nonlinear phenomenon which allows
some pressure waves to travel through air faster than the speed of
sound, surely a nuclear blast would produce enough pressure to excite
it. But it doesn't seem to. From
http://www.globalsecurity.org/wmd/intro/nuke-blast.htm:

"During the time the blast wave is passing through the superheated
atmosphere in the fireball, it travels at supersonic velocities. After
it leaves the vicinity of the fireball, it slows down to the normal
speed of sound in the atmosphere. As long as the blast wave is expanding
radially, its intensity decreases approximately as the square of the
distance. When the expanding blast wave from a nuclear air burst strikes
the surface of the earth, however, it is reflected, and the reflected
wave reinforces and intensifies the primary wave."

Roy Lewallen, W7EL