In a typical Southern California earthquake, with moderate to deep focus, the p-wave velocities can be in excess of 15,000 feet per second. This would be typical for a sound wave in crystalline rocks at depth. As the wave front moves into the sedimentray basin rocks, the velocity drops dramatically. In unconsolidated sediments, the wave velocity may only be on the order of 5,000 to 6,000 feet per second. Then of course, when a wave moves through a significant acoustic boundary (such as granite rocks to sedimentary rocks, or sedimentary rocks and the ocean water ... p-waves propogate through fluids ... sea quakes!) a percentage of the wave energy is reflected.
Seismology is a facinating subject. I sometimes wish I had continued on in an academic career in geology.