  Phonon spectra of detwinned {SrFe$_2$As$_2$} crystals, as measured by
inelastic x-ray scattering, show clear anisotropy accompanying the
magneto-structural transition at 200 K. We model the mode splitting using
magnetic DFT calculations, including a phenomenological reduction in
force-constant anisotropy that can be attributed to magnetic fluctuations. This
serves as a starting point for a general model of phonons in this material
applicable to both magnetic and non-magnetic phase. Using this model, the
measured splitting in the magnetic phase below $\it T_{N}$, and the measured
phonon linewidth, we set a lower bound on the mean magnetic fluctuation
frequency above $\it T_{N}$ at 210 K.
