Statement I: Compression and rarefaction involve changes in density and pressure.
Statement II: When particles are compressed, density of medium increases and when they are rarefied, density of medium decreases.

A compression is a region of medium in which particles come closer i.e., distance between the particles becomes less than the normal distance between them. Thus there is a temporary decrease in volume and a consequent increase in density of medium. Similarly in rarefaction, particle get farther apart and a consequent decrease in density.