On March 15, 2013, the National Diet Library made public their new digital archive of historical recordings. In partnership with a number of groups, including NHK, they have digitized and made available recordings from SPs from 1900 to the 1950s, in order to preserve them and prevent their becoming lost.
As time goes on, they plan to hold approximately 50,000 recordings in the archive. Although many recordings can be accessed via the Internet, some are only available to listen at the NDL itself due to copyright restrictions.
You can also access an NDL article on the digitization of recordings, entitled 音の歴史を残す (PDF link).
The archive is the Historical Recordings Collection, accessible at http://rekion.dl.ndl.go.jp/