A couple of items broadcast this week mentioned missing recordings;
It's been the 50th anniversary of the death of Kathleen Ferrier, and radio 3 played some unbroadcast recordings. sadly the only footage of her that exists was taken at an airport. you would have thought some broadcast/concert footage might have existed somewhere in the world.
Radio 4 mentioned that while the BBC has no recording of Orwell's voice,(He broadcast during the 2nd world war) a recording might exist, of Orwell narrating a Shakespeare play, which was issued as a BBC LP. The interviewed suggested it might turn up in a junk shop.