here direct from the horses mouth in acad 2000i
"Issue
WMF images created in AutoCAD appear to have missing objects when inserted into Microsoft Word.
Solution
This problem occurs if you create a WMF file in AutoCAD when the background color is black, and insert it into a Microsoft Word document when the background color is white. When the background color in AutoCAD is set to black, AutoCAD automatically changes the objects created with color 7 properties to white for easier viewing. This is true even when the system variable WMFBKGND is set to off (do not save the AutoCAD background color when creating a WMF file). Therefore, when you insert the WMF file into a Microsoft Word document with a white background, those objects that had their color changed to white, do not display.
For best results, set the background in AutoCAD to white before creating WMF files. This way, the color of objects with color 7 properties is black, and the objects display correctly in Microsoft Word."
So your answer is use WMFBKGND and set it to white if everyone helps everybody the world will be a better place