These images represent a wave.

which wave diagram is labeled correctly?

I believe the correct answer is the third one

The first picture is actually the correctly labeled wave diagram, as wavelength is the distance between two crests or troughs. Amplitude, however, measures the distance from the center line to the top of a crest or bottom of a trough.  

The speed of  sound:   sound travels  at different speeds depending on what it istraveling through. of the three mediums (gas, liquid, and solid)  sound waves travel the slowest through  gases, faster  through  liquids, and fastest  through  solids.