I know I said American folk music was on sufferance with me, but I have no problems with spreading the word.
The Roots Music Listening Room has a huge number of streaming files for your listening pleasure. Run by Juneberry78s.com, the listening room contains public domain music files from their collection of:
Rugged and Ragged Blues Piano from the 1920s
Blues from the 1920s (subset)
Gospel from the 1920s (subset)
Old-Time Country Music from the 1920s & 1930s
White Country Gospel from the 1920s-1950s
Vintage Hot Jazz from the 1920s
Louisiana Cajun Music from the 1920s - 1970s
Irish Dance Music from the 1920s - 1970s
Folk Music from Mexico 1920s - 1950s
Luther Strong's 1937 Library of Congress Recordings
Other Library of Congress Fiddle Recordings
Cape Breton Fiddle Music from the 1930s - 1950s
Calypso Music of Trinidad from 1930s - 1940s
S'free music. It's historic. Have at it!