Actually, I'm don't know for certain, but I'm sure it is possible. My uncle is an electronics genius, and one day, while repairing a TV, I complained that our home TV wasn't loud enough. He showed me an internal volume screw which was used to set the real volume at which the TV could go. When I asked him why they did that, he said that hotels, etc, need their TV volumes limited so that the next-door guests wouldn't get disturbed. So the hotels would set the max volume to a minimum
Anyway, I believe that the max volume that your powerbook is set at is due to what the built-in speakers can handle without distorting. If you want it louder, then you need to plug it into a good amplifier and use that to make it louder.
Curiously though, I've found that my Dull PC has a louder output signal than my Mac. But I've got a good amp, so this doesn't really matter. I'm sure that there is a way of hacking your Mac to get a stronger signal. But is it worth it when you blow your Powerbook speakers?