I know that institutions such as NLA and ABC hold huge archives of recordings of Australian history - historical music, news coverage of significant events, oral history interviews and so on.
Here's the NLA talking about some of their audio recordings
and here's some of the gems in the National Film and Sound ARchive
the USA has loads of their historical recordings available for display and re-use.
Here's heaps of old movie footage
here's heaps of old radio shows:
These materials are fantastic for education, for artistic re-use, for sampling, for research... but all those uses are prevented or made much harder by keeping them locked away on cassettes and films in the vaults of institutions rather than available online.