Happy Birthday is in the public domain now. As of a few years ago though, it was indeed legally murky though perhaps not unwise as long as performed in your basement with the lights out, and all in attendance are trusted and sworn to secrecy.

Hehe, well I might have, but Happy Birthday is now pretty safe, the US copyright was successfully challenged in court and the EU copyright expired. https://en.wikipedia.org/wiki/Happy_Birthday_to_You#Copyrigh...

It isn't anymore, if it ever was. https://arstechnica.com/tech-policy/2016/02/happy-birthday-i...