As David said, you wouldn't be able to use the managed C++ wrapper from C++/Cx, and whether you can use the unmanaged library depends on what calls it makes.
You can find the list of multimedia API which are allowed in Metro style apps at http://msdn.microsoft.com/en-us/library/windows/apps/hh452756 .
More information on the subset of Win32 and COM available to Metro style apps can be found at: http://msdn.microsoft.com/en-us/library/windows/apps/br205757%28v=VS.85%29.aspx