xs/tag.xs has in it

#include "tag.h"
 
MODULE = Audio::TagLib          PACKAGE = Audio::TagLib::Tag
PROTOTYPES: ENABLE
 
################################################################
#
# PUBLIC MEMBER FUNCTIONS
#
################################################################
 
void
TagLib::Tag::DESTROY()
CODE:
    /* skip if READONLY flag on */
    if(!SvREADONLY(SvRV(ST(0))))
        delete THIS;
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see "PACKAGE = Audio::TagLib::Tag"? That is where the method calls will wind up in. But since this is C++ XS which I'm not familiar with, not C XS, I think the "TagLib::Tag::" get stripped off.

As others said, your header() method in XS exists in a different class than you want it. Check the docs of your C++ library whether the TagLib::Tag class has a header method or not. According to http://taglib.github.io/api/classTagLib_1_1Tag.html it does not.

BTW, you should also mention you are the CPAN maintainer of Audio-TagLib ;)