Looking at it, I would say that you should be OK... In your copyrights section, you make it perfectly clear that files should be checked for copyright violation before submitting...
In essence this clears you from having responsibility... (or even knowledge that you were in violation)
Look at it this way - do YouTube pay a fine each time something is submitted and they have to take it down due to copyright? No - because users are told not to post anything that violates copyright agreements!