Yes, IIUC basically that's the point where the local particle speed is no longer sufficient to clear the leading edge of the wave.
Speed of sound isn't too directly affected by air pressure I think (holding all else constant): https://physics.stackexchange.com/a/146207
Isn't 460 m/s the speed of the shock waves?
I mean, it you make some "air particle" move, it'll hit a nearby particle, and so on, and this wave has a speed of 460 m/s. In a solid object (wood, for example) the particles are close together and the speed will be greater. If you remove air particles it'll decrease the pressure and the speed will be slower...