Well, looks like Apple have been moving things around again, and if you upgrade to Xcode 4.3 you're going to find that you can't build AU plugins any more because they've removed the old /Developer folder that contained all the headers.
What you now need to do is:
- In Xcode, choose "Open Developer Tool" / "More Developer Tools..", and download the audio package.
- Go to the Xcode.app, view its contents, and find the Developer folder inside it.
- Create an Extras folder inside the Developer folder, and copy the CoreAudio folder that you downloaded into it
- When you compile, you'll hit some errors in Apple's code which you'll need to fix. Nothing big, just add some casts and disambiguation for the latest version of Clang to accept it. (I had to do this in TextEdit because Xcode crashed when I tried to edit these files. Nice.)