Earlier quoted context omitted.
Everyone who has sung "Happy Birthday" is also a pirate. Yes, really. http://www.snopes.com/music/songs/birthday.asp http://www.unhappybirthday.com/ Sure, there's some evidence that they may have done some iffy things with the registration, but you'd have to go through an expensive trial to prove that, so good luck.
Why on earth is ryanklee's comment dead? He's right - it's not violating copyright to sing a song for non-commercial purposes. Hell, you can even cover a song in a paid live performance and not have to pay royalties. You just can't record it and sell the recording without ponying up. edit: looks like the live commercial performance thing is not true in the US, according to snopes. I guess those guys don't have cover…
http://www.musicianwages.com/the-working-musician/recording-...
"As a performer, you do not have to pay performance royalties. These are paid by the venue which hired you to perform. However, if you record your show and sell it as a live album, you will need to pay for the compulsory licenses, discussed later in this article."
Now, you could try to argue that it's innocent infringement (or maybe even fair use), but you have to do that in court.
From unhappybirthday.com:
"Did you know that if you sing any copyrighted song: ...at a place open to the public ...or among a substantial number of people who are not family or friends You are involved in a public performance of that work?"
For example, singing "Happy Birthday" at a restaurant....
EDIT: I had to turn on showdead to read that comment you mentioned. He mischaracterizes what Snopes said. I'll let Snopes speak for themselves: "performing a copyrighted work in a public setting such as a restaurant or a sports arena technically requires a license from ASCAP or the Harry Fox Agency (although such infringements are rarely prosecuted)." As you can see, the problem there is public performance not just commercial use.