Aaron Tiensivu has posted the commands you need to know to shut off the Windows speaker beep temporarily or for all time.

To temporarily disable the PC Speaker beep:

net stop beep

To disable the PC Speaker beep for good (until you undo this setting):

sc config beep start= disabled

Now you don’t need to run out of the room screaming next time you make a mistake at the Command Prompt that would normally make the hills come alive with BEEP BEEP BEE BEEEP BEEEEP. One liners: Disable the PC Speaker beeps in 200x/XP/Vista – Link