All the links here are broken:
XACT can be found here
Hope it helps
The links are not broken, or are not any longer anyway - they redirect to the Where is the DirectX SDK? page which tells us that the XACT Authoring Tool is no longer supported, that's why that part of the documentation is removed.
Where is the DirectX SDK?
The Xbox Audio Cross Platform Tool (XACT) is no longer supported for use on Windows.
As AbstractChaos answered, the XACT API reference is still there though but that in turn links to the removed documentation for the tool that is posted in the question. Hence, it's removed.
For detailed information about how to author audio using the XACT tool, including information about categories, variables, and other advanced features, see (redirected to discontinued support statement).
The wayback machine appears to have preserved a copy of at least some of the documentation:
You can download full MSDN Library for Visual Studio from the Microsoft website. But I think you need the same version of Visual Studio as is the version of the downloadable MSDN to be able to view it. Here is a link to download page of MSDN Library for Microsoft Visual Studio 2008 SP1, that should contain the information you are looking for. Unfortunately I don't own the MS Visual Studio 2008, so I can't make sure.