I don't think I'd agree that you can characterize the amplitude of a wave according to the density at only one point in it. After all ... a tiny wave in steel would be much denser at a compression than a huge wave in air would be.
The amplitude of any wave is described as the difference between a peak and the resting value. Or even better ... half of the difference between a maximum and a minimum.
So if you're looking at a longitudinal wave, like sound, I'd say if you want to describe its amplitude, then you have to look at the density at two points ... either the difference between the compression and the resting densities, or the difference between the greatest compression and the greatest rarefaction.
Since it went 100 meters in 10 seconds, that means it is going 10 meters per second. In 2 seconds, it must have gone 20 meters, if the speed is constant.