Hi Vijay,
Using a PARENTSRC for this isn't going to work, when you attempt to add the other xactengine3_* dlls. This will require multiple PARENTSRC which I'm not sure is even supported.
Since you only need three functions from xaudio_allocator.c, just copy them into xact_dll.c. It's unlikely they will change and will allow you to remove "#include "xaudio_private.h" and the PARENTSRC.
Regards Alistair