However, air bubbles in liquids have a profound effect on the speed of sound. The range of sound speed for solids is wider than for either gasses or for liquids.
Compressional propagation is the same mechanism as that in liquids and gasses.
Shear speed is VERY roughly equal to the compressional speed divided by the square root of three. I know of no fundamental limit at either the upper or lower limit for sound speed. Jet aircraft travel at supersonic speeds all the time. It was not always so simple, and the first time the sound barrier was broken no one really knew what was going to happen. Good luck with your project.