The multimedia keybindings are stored in
$HOME/.gconf/apps/gnome_settings_daemon/keybindings/%gconf.xml. But note that this directory only exists if you changed any of the values -- keys with their default value (e.g.
play = XF86Audioplay) are not stored. Default values for the multimedia keys are stored in a so called schema file (for details see the gconf documentation):
That said: If all you want to do is get the actions associated with a certain keybinding, you should never have to use the files directly. Instead, query the gconf database with a command like this:
gconftool-2 --get /apps/gnome_settings_daemon/keybindings/play
To get the default value (and other details about the key), you have to query the schema:
gconftool-2 --get /schemas/apps/gnome_settings_daemon/keybindings/play