Earthquakes are also compression waves. One of the things that I study is to compress crystals and see how the atoms respond. The ease in which a material can be compressed is related to the speed in which sound waves can travel through it. For instance, diamond is very incompressible, and sound waves, or compression waves, travel through it very fast. The speed of sound in air varies depending upon the temperature or density of the air.
Hot air has lower density and therefore sound travels slower.
Sound travels faster through cold air. Amplitude is not tied into the equation relating the other 3 wave parameters. It can be anything it wants to be. For sound, the amplitude of the sound wave is how loud the sound is. For an earthquake, the Richter scale measures the amplitude.