The UK based Open Music Archive ... is a collaborative project, initiated by artists Eileen Simpson & Ben White, to source, digitise and distribute out-of-copyright sound recordings. The archive is open for anyone to use and contribute to.
I've long been fascinated by old recordings for a couple of reasons: The pure fascination with history and the fact that old recordings can often be safely used in remixes as the Open Music Archives's about page describes:
Artists Ben White & Eileen Simpson have initiated this project following a series of projects which involved researching and gathering music which has fallen out of copyright. Much of this music, although legally in the public domain, is tied to physical media (for example gramophone records) and locked away in archives or private collections which are not widely accessible. The Open Music Archive aims to digitise as much of this music as possible in order to free it from the constraints of a physical collection."
Disclaimer: Copyright law varies between jurisdictions, and I'm not a lawyer, so mentioning this or other sources of audio materials here is not intended to be legal advice.