That's pretty much it =P
http://lasp.colorado.edu/cassini/images/Electromagnetic%20Spectrum.jpg
- points to the bottom of the electromagnetic spectrum -
A cellphone is a sophisticated radio, radio waves are at the bottom of the EMS. Our cellphones are emitting electromagnetic radiation! (oh, noes!)
http://electronics.howstuffworks.com/speaker5.htm
This will explain how positive and negative charges cause your speakers to make sound. As we know, electromagnetic radiation interferes with electronic signals (the reason that an EMP can shut off computers). It's not so much the magnet that is messed up by the EM waves, but the electrical current going to the speakers.
Speakers use so little electricity that something as weak as radio waves can affect it. For devices with a larger and more powerful flow of electricity, more powerful waves are needed.