AU is not working in 10.6

I’m using Osx 10.7 with Xcode 4.2.1, my AU plug-in works well in 10.7 but is not running in 10.6.
When I try to change the Base SDK from 10.7 to 10.6 I get errors in the AUBase.h, ComponantBase.h and AUPluginDispatch.h

What am I doing wrong here?

Thank you

Ok, I found the solution:

I’ve installed xcode 3.2 on another partition and copied it’s /Developer/Extras/CoreAudio/AudioUnits/
then I replaced my AudioUnits directory (Xcode 4.2.1) with the previous one. (ain’t that ugly or what…)

So now I have a few questions in mind:

  1. Why should anybody compile an Audio Unit with SDK 10.7? (it won’t be backward compatible).
  2. Isn’t any way to do this mapping within Xcode? if it has the option to compile with 10.6 then it should automatically supply the corresponding CoreAudio libraries, right?
    (Edit) 3. Do you think an Audio Unit should be backward compatible to more than the previous Osx version?