One simple answer is that electricity run through a loudspeaker makes sound. And physics tells us that changing magnetic fields cause electricity. You could run

One simple answer is that electricity run through a loudspeaker makes sound. And physics tells us that changing magnetic fields cause electricity. You could run that electricity through a speaker and "hear" the Earth's changing magnetic field. You would necessarily have to speed it up because we don't have 20,000 years to listen.