When in the command shell, it is too easy to press Tab to autocomplete too many times. I love the text auto complete feature — it saves my fingers — but I overuse it. This results in the annoying “beep” from the system buzzer. Mute it with the command:
$ /usr/bin/setterm -blength 0
You could also append it to the end of your .bashrc — assuming you’re using bash of course — so it runs every time you login.
If, like most Linux users you use Gnome Terminal or KDE Konsole, try exploring around, there are options to mute the system buzzer (or bell), and save the option to mute it as default.