This database with "60,000-plus tracks includes recordings from the world's greatest labels, including Hyperion, Bridge Records, Sanctuary Classics, Artemis-Vanguard, Hänssler Classic, Vox and many more. Coverage includes music written from the earliest times (e.g. Gregorian Chant) to the present, including many contemporary composers. Repertoire ranges from vocal and choral music, to chamber, orchestral, solo instrumental, and opera" (vendor's description).
http://imslp.org
A virtual library of public domain music scores (i.e., music no longer under copyright protection) which also provides a limited number of music recordings.
http://www.loc.gov/jukebox/
"...makes historical sound recordings available to the public free of charge. The Jukebox includes recordings from the extraordinary collections of the Library of Congress Packard Campus for Audio Visual Conservation and other contributing libraries and archives. Recordings in the Jukebox were issued on record labels now owned by Sony Music Entertainment, which has granted the Library of Congress a gratis license to stream acoustical recordings" (from website description). Currently includes approximately 10,000 recordings made by the Victor Talking Machine Company between 1901 and 1925 by such artists as Caruso, Sousa, Gluck, and Gershwin.
There are a few opera and musical theatre performances in this database which contains more than 250 definitive performances of the world's leading plays, together with more than 100 film documentaries, online in streaming video.