Public domain works are not protected by intellectual property laws and are free for you to use without permission. Some of these resources contain a mix of public domain and licensed materials--always check the specific license before using.
From the Creative Commons website:
"Creative Commons is a new system, built upon current copyright law, that allows you to legally use “some rights reserved” music, movies, images, and other content — all for free."
If you find music with a Creative Commons license, you must check the specific license to find out if and how you can use it.
Sources of sounds and sound effects for reuse. Be sure to check the license details for proper legal use.