It's in the User utility software on your PC (Assuming you have it installed)
Start > All Programs > Xerox > User Utility
Select the Scanner and OK
Look for Buzzer Setting
The Manual has it on page 182, and the User utility is here
Basically that. We're constantly scanning in my office, and the beep when scans are complete is really loud in this office. Is there any way to lower the volume in this model?