Roto-translational diffusion of uniaxial molecules has been investigated in smectic-C (Sm-C) phases. A modified version of the McMillan mean-field potential has been adopted to describe the tilted monoclinic symmetry of the Sm-C domains with respect to the Sm-A. The roto-translational dynamics have been modeled as a diffusive stochastic process, and the relevant first- and second-rank rotational correlation functions used to describe spectroscopic experiments (dielectric relaxation and magnetic resonance respectively) have been calculated. The model calculations show that some peculiar features of the relaxation processes in Sm-C phases emerge in the comparison of the same observables evaluated for phases with different symmetry, and this could suggest new and specific methods of analysis for the experiments on biaxial smectics.