How to use ESC key for Emacs

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http://askubuntu.com – When typing Esc within an Emacs window under Unity (Emacs 23 standard package), the sound is turned on/off, but Emacs does not receive the key. How can I change (probably) unity such that the Esc and also the other function keys are interpreted by Emacs? A very crude workaround that always works in (GNU) Emacs is to use C-[ in place of Esc. The question Why is the meta key is not working when using Emacs? does not apply because this was about using Emacs within a gnome-terminal, but I use Emacs directly under X. (HowTos)