It's complicated, food for lawyers:

http://www.djcounsel.com/copyright-101-an-%E2%80%9Cold%E2%80%9D-song-is-not-necessarily-in-the-public-domain/

If you use an existing composition, melody etc, which has been copyrighted by some author not dead yet, or dead since less than 75 years you are not set free. But the mechanical rights stay yours still if you recorded the stuff ...

Some copyright claimants are thieves themselves. Also modern ones. Stephen Foster for example 'stole' traditional material like Angeline the Baker and claimed it as nobody had done that earlier .... common monkey 'business' back then. You might get away with a lot of you don't make them copyright infringement hunters any wiser. And if you intend to use a known Bob Dylan song clearly mention that it's a traditional if it is originally.

As for PG generated sample material as in use of the musician recorded RT wavs it's free to use if you paid for the software.
A generous and gentile gesture, as some software music generation software and even prerecorded sample material's use is not so user friendly ... F