My music folder has a different icon from the other folders.
Gnome use the folder-music icon to personalize the music folder icon.
I would like to know where (in which configuration file or where in GConf, DConf) this association is made.
/etc/xdg/user-dirs.defaults (global) and ~/.config/user-dirs.dirs (local) contain the directions to default directories. The xdg-user-dirs-update command may also be of some use to you.
Edit: Sorry, I should have been far more specific. In gnome, the file manager Nautilus constantly monitors the XDG configuration files for changes, and is hard-coded throughout (for example; nautilus-icon-names.h) to match the specific pixmap to the specified XDG folder.