  We analyse the dynamical density fluctuation spectra for cuprates starting
from the $t-J$ model in a slave-boson 1/N representation. The results obtained
are consistent with diagonalization studies and show novel low-energy structure
on the energy scale $J+\delta t$ due to the correlated motion of holes in a RVB
spin liquid. The low-energy response implies an anomalous renormalization of
several phonon modes. Here we discuss the renormalization of the highest
breathing phonons in La$_{2-x}$Sr$_x$CuO$_4$.
