Some peculiarities of the crystals' vibrational spectra are studied in this work at non-zero temperatures and occasional fundamental frequency coincidence with the sum or difference of the frequencies of two other vibrations (temperature Fermi-resonance). The anharmonic interaction between the fundamental and the compound tone was shown to cause a renormalization of the spectrum also in the difference frequency region. The dielectric permittivity has been calculated in the temperature Fermi-resonance frequency region. The soft mode interaction with temperature excited difference tones in ferroelectrics has been analysed.