Free 2D, 3D, Audio & UI Assets by Kenney
These assets are free, public domain licensed (i.e. you can freely use in both personal and commercial projects without attribution), primarily for use with a Game Engine or Renderer, but you can always export to a Storyline supported file with an additional step or two (depending on the resource).
You can preview and download each individual pack here.
I could especially see the audio assets being tremendously useful to a few projects out there, already.