Yeah, speakers tend to wreak havoc on computers, even if you can't see anything happening. The bigger the speakers, the bigger the magnets usually are, and the worse it usually is for the computer. Although the size of the magnetic field is different with each speaker, the general rule is to keep electronics at least four feet from unshielded speakers. Most speaker magnets are "always on", whether or not they're plugged in.
The MBP probably has more magnetic shielding than other computers to protect them from the internal magnet that holds the power cable on. Still, if you'd like your computer to last a while, best to keep it away from external magnetic fields.
A few specialized speakers have their own magnetic shielding, but it can eventually fail. A quick test of the distance for a particular speaker is to use an old unshielded CRT TV; when the magnets stop interfering with the screen colors, it's about one third out of the danger zone.