Aluminium foil. It has interesting behavior in a magnetic field. This here works because, the foil is running through a magnetic field of a permanent magnet and through the foil an electrical current flows. Now the foil, as an electrical conductor excerts a magnetic field, pulsating to the amount of varying current, thus the magnetic field pulsates itself. Now the the magnetic fields of the permanent magnet and pulsating foil are repelling or attracting themselves, causing the thin foil to start vibrating according to its own magnetic field strength at any given time. Thus in return, the foil kicks air molecules up or down creating shockwaves... audible sound.
