Most recent version of the Terminal app here (0.2.5.7svn-). It, along with every previous version I have ever used, does not produce any audible sound from a pc speaker under normal circumstances. For example: xterm, aterm etc all beep when backspace is pressed on a blank command prompt. Xfce Terminal will not. http://forum.xfce.org/index.php?topic=2742.0
The audible bell is disabled by default. See the user manual for details (i.e. press F1 or read the manual online at http://www.os-cillation.com/documentation/terminal/C/advanced.html#hidden-options).