I've only recently started exploring the availability of public domain MP3s on the Internet, but it's a potentially fruitful line of inquiry. Archive.org has a haphazardly organized collection of public domain tracks, which is where I found the Mississippi John Hurt track linked to below. There's also a small collection at Public Domain 4U, but a high percentage of the links seem to be broken.
I remain quite fuzzy on exactly how public domain law works. When does a recording become fair game? Is there a reliable way of finding out if a particular recording is under the public domain? There are a few Mississippi John Hurt songs posted at Public Domain 4U. Does that mean that all of Hurt's 1928 recordings could actually be legally posted online at this point? Someone with some expertise in these matters, help me out. Write to me (firstname.lastname@example.org) with the subject line "public domain for dummies."