The velocity of seismic waves generally increases with depth in the Earth. This is due to the increasing pressure and temperature in the Earth's interior, which causes the density of the material to increase. As the density increases, the velocity of seismic waves also increases because the material becomes more rigid and resistant to deformation.
There are, however, some notable exceptions to this general trend. For example, in certain regions of the Earth's mantle, seismic waves may slow down due to the presence of molten material or other anomalies. Additionally, seismic waves can be strongly affected by the composition and structure of the material they are passing through, which can lead to significant variations in wave velocity at different depths.