[How to ] Kill the System Beep

System > Preferences > Sound > uncheck "Play alert sounds"

You can try a reboot now .. but. Disabling the PC speaker, in favor of using external speakers, can be more tricky.

Try by opening:-

1.sudo nano /etc/modprobe.d/blacklist.conf

.. and appending the lines:-

1.blacklist pcspkr

2.blacklist snd_pcsp

Reboot your Ubuntu PC.

If that doesn't cut it, open:-

1.sudo nano /etc/modprobe.d/alsa-base.conf

.. and add the lines:-

1.blacklist pcspkr

2.blacklist snd_pcsp

Reboot the box.