These are some of the works that would be in the public domain under the law that existed till 1978. That is, these are works that would no longer be subject to copyright protection.
Here is an article by musician Courtney Love about bands not making money. It was written in 2000 when file-sharing on the internet was still fairly new. It also touches on changes made to the 1978 copyright act in 2000. Some of these 2000 changes apparently were partially repealed by Bill Clinton later that same year.
Here’s a brief account of the original 1978 act by a conservative writer named Robert Unz. (Lest one think it’s only liberals who are critical of the status quo.)