DVD Deleted Scene

"Happy Birthday" is an expensive piece of music to use in a show or film, which is why it's often not used. Originally MGM did pay for the rights to broadcast the song in the episode "Childhood's End". However at that time very few TV programmes were released on video and DVD/Blu Ray hadn't even been thought of so the rights were not secured for that purpose. This means when the show is released on DVD rights have to be re-secured. This can often mean that DVD makers would have to pay again to use the song. In this case the rights were either refused or the price was too high and the originally scene has been edited to avoid the use of the actual song.