You might be interested in this ‘Citizen DJ’ project from the Library of Congress. It seems that they’ve picked only materials that are specifically free to use. The website is set up to make/scratch samples from these and mix them them sampled drums in-browser, but you can get to the source audio too.
Quote from this page Citizen DJ / Inventing Entertainment “The works in this collection have been identified to be in the public domain and are free to use and reuse without restriction. You can copy, modify, distribute and perform the works, even for commercial purposes, all without asking permission. Attribution is recommended but not required.”
For example: Citizen DJ / Inventing Entertainment
On this site is also page is a guide to reuse of other materials, not included in the project and may or may not be resuable.