It could be a ground loop. A normal ground loop happens if you have two grounded points in a circuit, for example if the speakers are grounded and the MBP is grounded, which would mean both would have to be using 3-prong cables. But speaker companies are smart enough to avoid this by assuming the computer will be properly grounded, and every other device will tie into the computers ground. If you are using the 3-prong extension cable for your charger, try the 2-prong adapter instead.
If you are already using the 2-prong adapter, the cause of the buzzing noise would be different. With the 2-prong adapter in the Magsafe, the computer is kept near but not at ground. The speakers should also be kept near ground by their 2-prong power supply. But the two "near" grounds are not the same, so current can still flow, and this may be picked up by the speakers' amplifier and made into the buzzing noise you hear. If this is the case, using the 3-prong Magsafe cord instead of the 2-prong adapter might help.
For further troubleshooting, also try disconnecting any other devices that might be plugged into the computers or the speakers.
If none of that works, you might be able to reduce the noise by improving the signal to noise ratio. To do so, turn the volume almost all the way up on the computer, then lower the volume on the speakers until it is at the setting you want. This is instead of setting the computer at a low volume then turning the speakers up louder. You could also try a ground loop isolator like this
http://www.radioshack.com/product/index.jsp?productId=2062214
but you will probably need a few adapters, and I don't know if the audio quality will be 100% preserved (ground loop isolators usually use an audio transformer, which is not always perfect). You could also try using the optical out and running it to an external amplifier, or getting some kind of digital (USB, Firewire) sound card.
