Arnold Schoenberg Center, Vienna. Extensive collection of Schoenberg's musical sketches, writings, photographs, sound recordings, and films. schoenberg.at You can find the sources for the Piano Piece Op. 33a here
Demonstration of Leon Theremin's Rhythmicon. Video on YouTube
Performance of Ruth Crawford Seeger's String Quartet, 3rd and 4th movements. The fourth movement begins at 3:45. Performance by Sarah Johnson, Anna Morris, Donald Schumacher, Richard von Foerster. Video on YouTube
Website for the Paul Hindemith Foundation. hindemith.info
Henry Cowell’s Musical Autobiography from 1961. Cowell talks about his musical influences and introduces his music. He discusses the Banshee and Fabric at 13:02. Video at YouTube