Songs in the Public domain are songs that:
- All rights have expired.
- The authors have explicitly put a work into the public domain.
- There never were copyrights.
For more information and an amazing directory of songs that you can you use, visit
https://www.pdinfo.com/public-domain-music-list.php
More:
https://www.mutopiaproject.org/
Here is a resource for choral works:
http://www.cpdl.org/wiki/index.php/Category:Sheet_music