I think that there has been some confusion on here because of the
differences
between Ubuntu and Xubuntu! In Ubuntu, the option exists under
System>Preferences>Sound to disable system beeps. Having run up an
Xubuntu live cd, I see that this doesn't apply.

I'm not sure if the following will sort it out for you but it's worth a try
if you haven't already. In a terminal type "sudo xfce4-mixer". This will
bring
up a mixer. One of the sliders is PC Speaker.