Bob - there are a couple of issues. The song may be in public domain, but the arrangement might be copyright. Even if both are public domain, you still have the biggeest issue - above all else, the PERFORMANCE rights still belong to the performer, unless you have evidence that you gave it away. If I recorded you 40 years ago singing "Mary Had A Little Lamb", LEGALLY I could not release or distribute that recording without your permission, or the permission of your heirs.

We've recorded artists for the radio station and received signed agreements. The agreements spell our our rights and the rights of the performer.

You can always go back and contact the surving members and their heirs.   If you cannot find them, keep records on your attempts to do so. You can probably make your archives accessible if you make the proper steps and try to obtain rights.

When you recorded the music, I assume it was done in the open and everyone knew that they were being recorded. What was the stated intent at the time?