We report on the first quantitative evaluation of the soft-mode behaviour of the relaxor PLZT 9.5/65/35. The transmission spectra of a PLZT thin film deposited on a sapphire substrate were measured in the region at temperatures from 300 up to 523 K, and in the range at 8 - 300 K. We fitted the spectra using the model of classical oscillators, and calculated the parameters of the observed lattice modes. The results were compared with those for bulk ceramics, and satisfactory agreement was achieved. The soft mode is heavily damped, and below room temperature it shows a strong hardening on cooling. Above room temperature, its eigenfrequency levels off, but the corresponding dielectric loss maximum continues to soften weakly down to at 523 K, where the mode becomes overdamped. The lower-frequency data require an additional relaxation below the measured range. Comparison of all of the available data at room temperature points to strong polydispersive losses in the Hz range, and another dispersion region at lower frequencies. We assign them to fluctuations of the polar nanocluster volume, and to reorientations of the cluster polarization, respectively. Their contribution to the low-frequency permittivity is much stronger than that of the soft mode.