An object under simple harmonic motion (MAS). Its maximum speed occurs when its displacement from the equilibrium position is

Newton's first law describes something special that results from the sum of all the external forces (including gravity) on an object. 

Sounds propagates through some kind of medium. usually it's air, but it goes just as well through solid objects like walls, windows etc. a soundwave will compress the (air)molecules in front of it, and if this increased density wave hits your ears, you register it as sound. solid objects like walls are a lot denser than air, this means that they will damp a wave, but also carry it easier.

D. the index of refraction of the second medium is lower.