The fantastic ‘radio happenings’, discussions between John Cage and Morton Feldman:https://archive.org/search.php?query=john%20cage%20and%20morton%20feldman
A great collection of recordings and interviews from the Other Minds series. https://archive.org/details/other_minds
http://www.ubuweb.com A fantastic collection of everything avant-garde.
The complete writings of Gertrude Stein: https://monoskop.org/media/text/stein_complete_writings/
Great site on everything Feldman http://www.cnvill.net/mfhome.htm
Video game sounds: https://www.soundsnap.com/tags/video_game?page=1
Found home recordings and other tape deck oddities: http://www.sweetthunder.org/tapes/weekninety.html