Perceived-pitch tracking of potentially aperiodic sounds, as well as pitch tracking of multiple simultaneous sources, is shown to be feasible using a probabilistic methodology. The use of a shift-invariant representation in the constant-Q domain allows the modeling of perceived pitch changes as vertical shifts of spectra. This enables the tracking of these changes in sounds with an arbitrary spectral profile, even those where pitch would be an ill-defined quantity. It is further possible to extend this approach to a mixture model, which allows simultaneous tracking of varying mixed sounds. Demonstrations on real recordings highlight the robustness of such a model under various adverse conditions, and also show some of its unique conceptual differences when compared to traditional pitch tracking approaches.