When a cell phone begins receiving a call, or checks in with a nearby tower, the phone's RF receiver is turned on and off repeatedly at a very fast rate. This ‘pulsing' can be picked up by nearby devices that do not have proper shielding. The phone is actually required to transmit at a higher level the farther away it is from the nearest tower, which may translate to a louder buzz.
New technology is changing the way this transmission occurs, and there are now standards in place for manufacturers of equipment like your computer speakers to ensure that they build their products with enough shielding to prevent this buzz from occurring. In the future, the cell phone buzz may be a thing of the past!