Updated the rules to say:
"You may use any library, game engine, tools, art, or audio to develop the game, provided you have the appropriate license to do so (if licensing is required for what you use); note that this means you may use code, art, and audio that you or others may have made in the past (the explicit limitation is that you may not use existing game logic (i.e. you may use libraries and engines, but you still have to actually make the game during the 7 day period))"
I think there are tons of freely available resources for art and audio such that if you have a few of your own that you've made, it won't give you an unfair advantage. Plus your game must fit the theme that will be announced, which means someone will either have to be very creative about how they use existing assets, or use new ones specifically suited for the theme.