We do not allow any copyrighted material on newgrounds with or without credit. As a general rule, if you did not create it, it does not belong here.
The only exception is sampling/remixing content released under the Creative Commons License. For the most part this means other stuff on Newgrounds. If you were to choose to rap over an instrumental that was made by another Newgrounds member, then that is fine (Although you must still give credit, and I strongly suggest you ask first).
As mentioned above, should you use copyrighted material in a submission, you will be held accountable. In general, we do not give warnings, as you are expected to understand and follow the rules before your first submission. Should you break those rules, you will be banned, and audio bans are permanent.
The More You Know!