If you really want to be safe on whether a song is public domain or not, if you were to add 190 to the year the song originated, or to the year you knew it existed, that would be the year that it is 100% not going to be in copyright unless they change the laws again.

What math is this? In the USA, they are saying life of author plus 70 years. Well, if we don't know if someone is alive or dead, take 120 years (the max anyone usually lives anymore) + 70 years and get 190 years. This means that in 2090 all songs from 1900 are definitely in public domain. We use this number, when we don't know when the copyright owner died. It also means that any song from 1978 will definitely be in public domain in 2168. This also means that all music from 1832 and earlier, is definitely safe.

I am unaware of any music copyright owner playing the tricks Disney did to keep their movies in copyright. This would be the only possible issue. With that in mind, I would assume that any song or piece that was in a Disney movie that did not exist before the Disney movie, is in copyright regardless of any math! I would also have the same logic for other movie and tv show production companies just to be on the overly safe side. They have lawyers who are extremely Ace at finding loopholes to laws that the Judges tried to sew up.