Why is there no sound coming through the speakers attached to the monitor?

I have a Dimension 8400 which came with a sub-woofer and 4 separate little loudspeakers.  The monitor also has two loudspeakers mounted under the flat-screen monitor with the volume control in the middle of them, but I have never heard any sound coming from them.  The subwoofer and 4 loudspeakers work fine, but take up a lot of space and tend to get knocked over so are quite frankly far more trouble than they are worth.  I would much prefer to unplug the separate speakers and to use only the ones attached to the monitor.   I assume, however, that I have to have the subwoofer attached as there is a cable coming out of the back of the monitor, so I have attached the cable (with orange end) to the subwoofer, and the subwoofer is plugged into the back of the computer (green, black and yellow ends).  However, I have left the 4 individual loudspeakers (Front L R and Rear L R) unplugged.  All I can get is some not very inspiring sound out of the subwoofer.  There is no sound coming out of the monitor speakers at all.  Are they just there for decoration, having been disabled by the manufacturers because the computer was supplied with the other loudspeakers?  Or is there a fault?  Do I have to have all 4 loudspeakers and the subwoofer plugged in to get the correct sound?  Or is there something I can click on in Windows which will allow me to use only the monitor's speakers?  I've had a look in the Advanced Speaker Settings in the Volume Control menu but I can't seem to make any difference.  I would be grateful if anyone could help.