Hi online1,
First of all, consider every component in the signal chain, both real and virtual. Make sure the volume is turned up on your source component, whether it be your PC or your mp3 player. Check your Windows Volume Control (Start>Accessories>Entertainment>Volume Control), and make sure that your master volume is turned up, but also check the 'Wave' and 'CD Player' volume levels. Then, once you've checked every possible volume control on the source component, look at the G51 speakers. Of course, be sure they're powered on, but then turn up the volume control. Also, check the 'Mute' button on the control pod. If the small amber LED beside the Mute button is illuminated, press the button until the LED turns off.
Remember, these are PC speakers, meant to produce listening levels up to 85 dB or so at a distance of 1 meter from your PC workstation. They shouldn't be capable of setting objects in motion throughout your house. Finally, if none of my suggestions set you straight, try returning the G51s and exchanging them for another set. I know that my own set of G51s (which I still use and enjoy) are capable of going quite loud, though if you happen to have very severe hearing damage (to the point where you cannot hear people speaking beside you), you may not be able to get significant enough levels from the G51 system to produce the same experience you may have, say, from standing in front of a full stack of concert bass bins.