Yes, assets include audio, visuals, code, shaders, text - anything that goes into a game. As long as it's freely available (meaning anyone could use it) then you can use it in the jam.
But note that we limit any related score for Judges Choice to 4/5, so you can only score 5/5 for Graphics/Animation if the visuals are original - and you will be required to list any assets which are not your own, which may also affect your Developers Choice score.