While the acoustic analysis idea may* work, I think the accellerometer data would be much easier to analyze. If you are looking for something that can give you better rates of data, try looking into other accellerometers. [This][1]This one from Adafruit has a maximum rate of 800 Hz([datasheet][2]datasheet). According to your calculations, this should be enough.
You could log this data with something like an arduino to be analyzed later (I haven't looked into how quickly that will be able to gather/process data). A module like the raspberry pi zero should have enough processing power to sort through the accellerometer data in real time.
As for measuring the speed of the skateboard, a common, cheap, and fairly easy route is to use a [hall effect sensor][3]hall effect sensor on a wheel. This will give you the angular velocity of the wheel (rpm/rps) and from there you use rpm times the circumference of the wheel (say in centimeters) to give you the speed in cm/min. Convert appropriately for your program.
*I'm sure given enough time, trial and error, it could work, I am doubtful using sound to analyze the road would be worth the effort. [1]:https://www.adafruit.com/products/2019 [2]:https://cdn-shop.adafruit.com/datasheets/MMA8451Q-1.pdf [3]:https://en.wikipedia.org/wiki/Hall_effect_sensor
