I didn't mean to give you the wrong impression. Most government things are in the Public Domain. Also, things like the Forest Service, BLM, Fish and Wildlife are taxpayer funded and in the Public Domain. With that said, though, certain images/audio on Public Domain sites may have been taken by a professional photographer/audiographer who wants copyright credit. I have certainly found that to be the case on a lot of Public Domain sites.

A just because you find Public Domain material on a Public Domain site, doesn't mean you can follow a link from said site to another and the same rules apply. Looking at Terms of Service on any site, particularly Public Domains sites, informs one that you are be redirected to another site and to check that sites polices.

It's been several years, since I found the trolley sounds, but beleive it was the trolley museum site. It was a private entity, not public domain. Usually in those cases, I review the websites copyright or terms of service. As I use a lot of stuff off the net, I also keep emails and copies of copyright permissions. It's entriely possible I got them from a free sound site, but as I recall it was the museum.

As for images off Public Domain sites, a lot of them require some form of credit, and not a Creative Commons License. If you can find an item that is non-copyrighted, they uslally have the photographers name and date, or give you a catalog number to use as a credit such as LOC-N-1234-567 Delta Queen or Image Credit-John Doe-1865.

With that said, an itme that has lost it's copyright can be copyrighted again, but another entity. For example: say I found a non-copyrighted image off LOC and put it on my website. If you download the image from my website, you are in violation of my copyright rights.