From a fascinating report by the US Council on Library and Information resources. Here’s an excerpt from the summary:

Survey of Reissues of U.S. Recordings finds that most U.S. historical sound recordings have become virtually inaccessible—available neither commercially nor in the public domain. According to the report, the rights to 84 percent of historically significant recordings made in the United States between 1890 and 1964 are still owned by someone and are therefore protected by law. For most pre-1972 recordings, protection comes in the form of state, not federal, law until 2067. Because recordings cannot be copied and distributed without permission of their rights holders, the only legal way to obtain a CD of a pre-1972 recording is through a reissue. Yet the study found that rights holders have reissued—or allowed others to reissue—on CD only 14 percent of the pre-1965 recordings they control. Thus, most historically important sound recordings are available for hearing only through private collectors or at research libraries that collect our audio heritage and have the equipment to play obsolete, often-frail recordings.

And the significance of this? Simple: it provides evidence for the claim that the current ‘strong’ copyright regime keeps a large proportion of creative works inaccessible.