Under versions of Emacs before 18.58, the bell volume was annoying loud and difficult to turn off. So upgrading to 18.58 will reduce the volume. If you want to turn it off completely, use `xset'. There is no way to turn the bell off just for Emacs without affecting all other programs.
Under Epoch you can do:
(setq epoch::bell-volume 20)
Stu Grossman <email@example.com> wrote a patch that allows the bell volume to be adjusted from inside Emacs just for Emacs.