Copyright laws are simple in theory but complicated in application. I am no lawyer but I think that you may have to seperate the song from the collection.
Example: A broadsheet song published in 1880 may be a long time in Public domain. If a folksong collector hears someone in a Newfoundland outport singing the song in 1980 and publishes it in a book does he own the rights to the song. No, but he owns the right to copying it from his book. If you got the song from another source he can have no claim. However, when you say the song is similar do not use the "similar" version. Is the collection of the "someone else" in public domain? Even if not the publisher waived most rights by using "author unknown".
Crown Copyright in Canada has been an area of dispute for many years. The government holds the copyright on behalf of its citizens. The argument is then do the citizens have the right to copy it?
Copies are usually allowed for non commercial use provided that the copy is not changed and creditation is given.
Copyright in Canada remains until the last author dies plus 50 years. Published crown copyright works are in copyright until 50 years after they were first published. If the author of the copyright assigned the work to the Crown, then it would go into public domain 70 years after the authors' death. Unpublished Crown Copyrights do not enter the public domain until 125 years after the title was created.