A few useful links concerning public domain music...
Here is a great page of easy to understand, general guidelines and information about public domain vs. copyrighted music from Wiki:
https://en.wikipedia.org/wiki/Public_domain_music
Another very good source of quick general info:
And here is their very cool searchable list:
https://www.pdinfo.com/public-domain-music-list.php
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A couple other search sites that can point you to public domain/use-able/free music: