C sharp/D flat Major.

If you need a version of the song without the lyrics to practice singing to, here's a piano cover I did of the song. It's not a flawless recording, but it might help.

Hmm....well I ran through some karaoke sites I use to download music, which usually tells the key signature of a piece, but unfortunately, I didn't find any karaoke recordings for Can't Take It In. Now, I have the LWW Official Score & Vocal Sheet Music Book that came out with the film and it has the song here. Looking at it, and being a vocalist/piano player myself, I am almost certain that the song is in C sharp, since there are no sharps or flats following the key signature. Had I a piano at the moment, I might have been able to play through it to make sure it's really in the C sharp key. I believe it is though!

If you'd like, you could also ask fellow NWebber, MinotaurforAslan, because he did a piano cover for it on youtube, which he compiled himself and would probably be able to give you a hand with this as well.

Hope that helps! Oh, and I love that piece so much!

Edit: I guess Mino already answered this question while I was typing!

Since it seems that the question has been answered, I'll go ahead and close this thread.