  An approach to calculate high-frequency bulk and shear modului of
two-dimensional (2D) weakly screened Yukawa fluids and solids is presented.
Elastic moduli are directly related to sound velocities and other important
characteristics of the system. In this article we discuss these relations and
present exemplary calculation of the longitudinal, transverse, and
instantaneous sound velocities and derive a differential equation for the
Einstein frequency. Simple analytical results presented demonstrate good
accuracy when compared with numerical calculations. The obtained results can be
particularly useful in the context of 2D colloidal and complex (dusty) plasma
monolayers.
