  We consider the effects on phonon dynamics of spin-lattice coupling within
the slave-boson mean-field treatment of the extended $t$-$J$ model. With no
additional assumptions the theory is found to give a semi-quantitative account
of the frequency and linewidth anomalies observed by Raman and neutron
scattering for the 340$cm^{-1}$ $B_{1g}$ phonon mode in $YBa_2Cu_3O_7$ at the
superconducting transition. We discuss the applicability of the model to phonon
modes of different symmetries, and report a connection to spin-gap features
observed in underdoped YBCO. The results suggest the possibility of a unified
understanding of the anomalies in transport, magnetic and lattice properties.
