[link] Watch it until the end, where you can hear the sound very clearly.
After seeing the theory that you could hear songbird's cry while at Fort Frolic, I decided to take a closer look at it. Other videos I saw weren't clear enough, and for the most of it I wouldn't believe unless seeing it by myself.
You can clearly hear a screeching sound multiple times. You could say the sound play too many times, or that Songbird doesn't actually scream like that, but nonetheless it's funny how the developers planned this easter egg so ahead in time.
Want to check it by yourself? Get this save file: [link] It places you right at the elevator where this video starts.