The only way you can override the sound setting is to change the volume yourself, do the beep, then put the volume back where you found it. There is no little magic "override" method you can call.
The big problem with the is that you're changing a setting the user has set for a reason. THe other one is that if you turn the volume up to some arbitrary level, you have no idea how loud the sound really is. It still might not be audible or you might blast the user right out of their chair!
Along with what the others say, the user may have the speakers physically turned down or off. Or headphones could be plugged in but the user not wearing them. There is also the possibility that the user is remoted into a system halfway 'round the world.
Perhaps you could allow the user to set a configuration setting to specify the volume to use.
For one application I wrote for a former employer I allowed the user to specify what sound to make (if any) as an alert.