https://en.wikipedia.org/wiki/P-wave P-waves from earthquakes are very low frequency sound waves.Typical values for P-wave velocity in earthquakes are in the range 5 to 8 km/s. The precise speed varies according to the region of the Earth's interior, from less than 6 km/s in the Earth's crust to 13 km/s through the core.
well not entirely, as in the earth, higher density will generally be directly proportional higher pressure, so no problem there
The velocity of sound has a very simple functional form:https://en.wikipedia.org/wiki/P-wave P-waves from earthquakes are very low frequency sound waves.
Sound waves are in fact compression waves: http://www.physicsclassroom.com/class/sound/Lesson-1/Sound-is-a-Pressure-Wave
See Birch's law : https://en.wikipedia.org/wiki/Birch's_law which holds for materials not under enormous pressure. Which seems to be somewhat at odds to what @TeethWhitener indicated.