Yep, the Internet Archive is a humongous resource. The thread title is a bit off, though: if there's a Creative Commons license in place, chances are that the material is not freely in the public domain, but some rights are reserved. Material is truly in the public domain if it's free to be used without any kind of restriction.

That being said, a lot of the material being released under a CC license scheme is generally favorable in regards to material reuse (like sampling).