I guess that it's Chuck Walbour, given how evasive the answer is.
The latest SDK is always intended to include all the files needed to develop for supported platforms. DirectX SDKs are not shipped as "patches" or "updates." Thus, for the files to be missing means either that:
1) X3DAudio is deprecated
2) someone forgot to include them in the SDK redistributable
It sort-of kind-of sounds like you're saying that the actual answer to the question "what happened to X3DAudio" is 2), but given that Chuck cannot, ever, admit to anything that might look like a wrongdoing on the part of Microsoft, he will try to deflect any discussion about whether these files were accidentally or purposefully omitted or not.