XBMC uses the default libFLAC.dll from the win32 developer archive from flac.sourceforge.net

The current version is 1.1.4, and the sources may be downloaded here:

http://downloads.sourceforge.net/flac/flac-1.1.4.tar.gz

The current binary dll is found in the archive available here:

http://downloads.sourceforge.net/flac/flac-1.1.4-devel-win.zip