It is simple. The higher the density, the slower the wave. If you are talking about waves that are on the Electromagnetic Spectrum (radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays), then the speed in a vacuum is the same as the speed in the air. A solid is more dense than a liquid, so the wave will travel slower through a solid, and then vice-versa.
Hope this helps buddy :-)