We have measured phonon spectra of Ag₃M(CN)₆ (M=Co,Fe) using neutron time-of-flight spectroscopy as a function of temperature and pressure. Ab-initio calculations were carried out for the sake of analysis and interpretation. Thermal expansion has been calculated via the volume dependence of phonon spectra. Our analysis indicates that Ag phonon modes within the energy range 2 to 5 meV are strongly anharmonic and major contributors to thermal expansion in both systems. The application of pressure hardens the low-energy part of the phonon spectra involving Ag vibrations and confirms the highly anharmonic nature of these modes.