You may! Third party assets are allowed (not just CC, but also paid or otherwise licensable) if they re publicly available to everyone to use or purchase.
Quick note for everyone reading: The spirit of this is a level playing field for participants, so while it's allowable by the letter of the rule, some obvious loop holes are frowned upon. One such example of such a loop hole is making your own assets CC just prior to the jam starting and saying they're publicly available on your own website.