If a song's music & lyrics were published before January 1, 1923, then the song is considered in the Public Domain in the US. Please visit pdinfo.com to learn more about Public Domain works and see a list of songs that qualify. It's important to know that if a work falls within the Public Domain, only that exact version is not protected under US copyright law. Other people may have created arrangements of that work that ARE under copyright. Do your research!
If you are an international artist: keep in mind that CD Baby is a US based company and we must follow the public domain considerations for the United States. Though a song might be in the public domain internationally, it may not be in here in the United States. If this is the case, CD Baby cannot consider the title public domain but rather it would need to be marked as a cover song due to our copyright laws.