Synfig supports a multitude of layers of various types; geometric, gradients, filters, distortions, transformations, fractal and a few others.
Full-featured bone system allows to create cutout animation using bitmap images or control your vector artwork. The Skeleton Distortion layer allows to apply complex deformations to bitmap artwork.
Synfig have basic features for syncing your animation with soundtrack using Sound Layer. Linux users can get all benefits of full-featured audio editors through the JACK integration.