There are 2 ways to assign a multimedia function to an arbitrary key (on the keyboard) xmodmap and gnome-control-center/keyboard/shortcuts. So, I'm trying to understand how they interact, i.e., does gnome-control-center insert its mapping to Xorg or into GTK.

I've experimented by these tool: "xev" and "key_tool". It is clear that the these tools give the same key data under different names. Here is the corresondence:
key_tool | keyval | hardware_keycode
xev | keysym | keycode

If I map the Pause key to XF86AudioPlay by xmodmap, I see
keycode=127 (Pause)
keysym=0xff13 (XF86AudioPlay)
Also, Mplayer reacts to Pause.

If I map by gnome-control-center, I do not see keypresses at all, Mplayer does not react to Pause. But key presses are not discarded because Audacious reacts to Pause as expected. Also, if Audacious is not running, I see a big ∅ sign when pressing Pause.

So, gnome-control-center inserts its mapping to Xorg but it maps the key press to another event. The question is, how and what event?