It's not the earphones. If a magnetic field was strong enough to stop speakers or your earphones from working, it would probably yank the earphones off your head :lol:
What's happening is probably what someone electro said. You're getting interference from your welder in your MP3 unit. The simplest solution to this problem would be to get a longer headphone cord. Also, make sure there are no electronic devices other than speakers on your headphone or along the cord to the headphone, like a pause/play button.