A speaker out of fischertechnik. The membrane is a 250 g quark box. A neodym magnet on the inside and one on the outside of the membrane hold each other in place. The membrane is driven by an old fischertechnik magnet.
The impedance of the speaker is 50 Ohm. It can be connected to any speaker or haedphone jacket. The sound might be somwhat quiet, though. It is easy to find KZfaq videos that show how to build specific amplifiers for an impedance of 50 Ohm.
The recorded audio signal in the video comes directly from the headphone jacket of the World Receiver without any further amplification.