well, north could change locally, but the magnetometer points to it

i can't picture a way you would be able to shield a regular compass from a close magnet, EM radiation, sure, but not plain magnetic field, though if the magnetic field is always at an exact offset from the magnetometer, you can calibrate it to ignore it (that way you can get rid of issues with speaker magnets etc, but depending on the wiring, it might still go crazy when the bass go bumping)