The whole site looks like it deserves some time for exploration. Here's how they describe the site: "A reference site to help identify public domain songs and public domain music . . . lists of public domain music, royalty free music recordings you can license, and public domain sheet music reprints . . . precisely the music you need for your business, organization, or personal use."
Here's how they describe the project: "The Public Domain Information Project was started in 1986 by Marji Hazen of Ashland, Ohio. Marji was particularly interested in folk music and began making a list of public domain folk songs. Over the years, the list expanded to include any PD music and Marji collected nearly 10,000 PD songs from libraries, used book stores, folk publications, and just about any other published source imaginable.
After the GATT Treaty and 1998 Sonny Bono Copyright Extension Act added new complexities to PD research, Marji took the PD Info web site off of the web while she looked for a new webmaster. In early 1999 Haven Sound agreed to take over PD Info and keep the Public Domain Information Project alive and growing. Haven Sound is affiliated with a recording studio and audio post facility that works with young musicians and provides music and sound for independent film and video. Haven Sound's path crossed Marji's while looking for public domain music to use for independent film sound tracks with no budget for music. "