I've never actually used public-domain media... but I am familiar with some of the online archives.
None of them guarantee that the media is 100% in the PD, and some of them are better than others at actually verifying what they collect. Many of the sites accept uploads from individuals that may (or may not) be verified by the archivists. They get a lot of new material daily.
Some of the archive sites (such as archive.org) add media to the collection that is copyrighted, but authorised for archiving/exposure by the content creators. That can lead to confusion among people browsing, especially when different parts of the site will have different rules and regulations for different categories.
Be careful.
If you want to be extra cautious, you might want to look at E&O insurance. (Errors & Ommision) When shopping your Zappa flick for a distributor, they may well require it anyway. I have no idea how much it runs (I make crap; not worthy of it) but you should definitely check.
:shock: