You can use assets that you create in Blender. You can also use assets you create in other programs like Maya or Plasticity (paid programs). The point is that you are not to use assets you spent money on directly.
Edit: Some services like MotionArray.com have subscription fees that open up libraries of content like sound effects and music, among other things. Even though you don't pay directly for specific assets, because you are paying to get access to a curated library of assets, I don't think you can use those, though you might have to ask the organizers about that to be sure, if that's something you'd consider doing.