Ferromagnetic resonance studies and magnetoelastic coupling of a set of epitaxial Fe${}_{1\ensuremath{-}x}$Si${}_{x}$ films on GaAs(001)$c$(4$\ifmmode\times\else\texttimes\fi{}$4) are reported. Fe${}_{1\ensuremath{-}x}$Si${}_{x}$ alloys form a solid solution for low Si concentrations and atomic ordering at the composition of the Heusler compound Fe${}_{3}$Si. The provided magnetic anisotropy constants are discussed with respect to the growth parameters. The high uniaxial in-plane anisotropy is related to the interface as evidenced by its thickness dependence. The contribution of the magnetoelastic coupling to this uniaxiality is low. For layers grown at 250 ${}^{\ensuremath{\circ}}$C, the formation of a two-phase system is indicated by the dependence of the cubic fourfold anisotropy on the Si concentration. The resonance linewidths are less than 2 mT, thus corroborating high magnetic and crystallographic quality. The angular out-of-plane dependence of the linewidth is explained by a contribution from two-magnon scattering; the unusual symmetry observed in plane is discussed in the framework of the diverse relaxation mechanisms.