Digitization of historic sheet music, musical manuscripts, and other primary music resources continues to advance rapidly. Here is a selection of digital archives that could be useful in your research.
Featuring choral music from the collections of the Library of Congress, this resource is a result of collaboration between the Library and the American Choral Directors Association.
483 reel tapes from Juilliard's first decade of audio recordings, 1951 to 1959, are available online. Featured performers include John Cage, Isaac Stern, and Risë Stevens, as well as student performances by Van Cliburn and Leontyne Price.
Duke University's Historic American Sheet Music Project provides access to digital images of more than 3,000 pieces published in the United States between 1850 and 1920.
Instrumental parts for over 3,500 compositions, including markings, marginalia, and manuscripts in the hand of Bronislaw ("Nek") Mirskey (1887-1927). Mirskey worked as music director at several theaters in the U.S. between 1914 and 1927. The collection is held by the University of Pittsburgh.
The New York Public Library's collection of over 700,000 digitized archival images, including prints, photographs, maps, manuscripts, and streaming video.
Links to over 100 sites compiled by the Harvard College Library, including archival collections, online scores and sound recordings; article indexes, discographies and bibliographies; scholarly societies; and musical reference works, most of which are accessible to users outside of Harvard.
Over 21,000 digitized scores and 1,500 music books in the public domain available through the library at the University of Rochester's Eastman School of Music.
List of over 60 sources that offer "access to collections of digitized music documents (typically originating from printed or manuscript musical sources). Some contain scanned images, some contain fully encoded scores, some contain encodings adapted for music playback (e.g. via MIDI), and others (e.g KernScores) are adapted for music analysis" (Wikipedia).