Using songs like Happy Birthday or non PD pop songs or whatever costs a lot to do legally. Thats why some tv shows or movies make up their own happy birthday baloney song so they dont have to pay. You hear this on some dvd commentaries how they pay through the nose for those small snippets you hear sometimes. Into the tens of thousands.
Pop goes the weasel or camptown races on the other hand.. public domain, so go nuts. Happy Birthday however? Pay up or be sued.
http://en.wikipedia.org/wiki/Happy_Birthday_to_You