  We calculate the full density response function, and from it the
long-wavelength acoustic dispersion for a two-dimensional system of strongly
coupled point dipoles interacting through a 1/r^3 potential at arbitrary
degeneracy. Such a system has no RPA limit and the calculation has to include
correlations from the outset. We follow the Quasi-Localized Charge (QLC)
approach, accompanied by Molecular Dynamics (MD) simulations. Similarly to what
has been recently reported for the closely spaced classical electron-hole
bilayer [G. J. Kalman et al. Phys. Rev. Lett. 98, 236801 (2007)] and in marked
contrast to the RPA, we report a long-wavelength acoustic phase velocity that
is wholly maintained by particle correlations and varies linearly with the
dipole moment p. The oscillation frequency, calculated both in an extended QLC
approximation and in the Singwi-Tosi-Land-Sjolander approximation, is invariant
in form over the entire classical to quantum domains all the way down to zero
temperature. Based on our classical MD-generated pair distribution function
data and on ground-state energy data generated by recent quantum Monte Carlo
simulations on a bosonic dipole system [Astrakharchik et al, Phys. Rev. Lett.
98, 060405 (2007)], there is a good agreement between the QLCA kinetic sound
speeds and the standard thermodynamic sound speeds in both the classical and
quantum domains.
