According to this discussion on TechNet and me checking both Windows 8 and Windows 8.1 locally, the 'Non-Plug and Play Drivers' are no longer accessible in Device Manager in 8.x regardless of what you try.
You can always just try to disable the beep driver directly:
change the Start value from 1 to 4, which means disabled.
.REG registry file would be:
Windows Registry Editor Version 5.00
You have to reboot your machine.
I haven't tested this myself, but disabling the beep driver should not break anything else in your system.