It’s amazing how quickly the blast filled the whole valley. I suspect the blast front travelled at the speed of sound. I’m used to seeing blast videos in slow motion.
During development it was also determined that the overall effectiveness of the GBU-43 can be very much dependent on the ambient weather conditions. Cloudy humid weather tends to diminish the blast effect.
To get a little technical, because this is FR and you made a good guess, you're close. The shock wave (thickness 0.0001 inches or less) travels at what can reasonably be treated as a weighted average of the speeds that sound would travel in the media on either side of it, accounting for what are initially very different densities and temperatures. For "short" distances from the blast, the shock wave is supersonic relative to the speed of sound in normal air. At the radius where the wave goes subsonic, the nature of the sound changes, not that anyone near the MOAB would notice. At long distances, the blast wave slows down a lot before the movement reverses direction as the return wave fills in the partial vacuum left by momentum and over-expansion.