Existing code libraries and engines may be used, whether or not they were made by you (but games or game logic may not be). As for art... I'm not sure. A part of me wants art to be original (and made during the 7 days), but at the same time, I don't think I'll have require audio to be made/composed during the 7 days... My current thinking is that 2D and 3D art must be made during the 7 days, but audio is not restricted. Thoughts on this?
Seems like for art and music assets, a flat "create it within the 7 days, or use public resources available to everyone" is fair.