FM Sound Unit
From Sega Retro
The FM Sound Unit (FMサウンドユニット) is an add-on for the Sega Mark III used to enhance the sound output of certain Mark III games. It was sold in Japan from 1987 for ¥6,800.
The FM Sound Unit contains a YM2413 FM chip that acts on top of the built-in SN76489 PSG chip, adding nine extra mono sound channels. When the redesigned Sega Master System hit the Japanese market, the FM Sound Unit was built-in to the console (though it is absent from international releases).
Unfortunately not all Mark III or Sega Master System games made use of this feature. The unit was not built into the Sega Mega Drive or Power Base Converter, so these extra features cannot be activated when playing Master System games on Sega's 16-bit console.
(Note in most cases, only the Japanese variants are compatible)
The following games have FM soundtracks but were not released in Japan:
Out of these, only Wonder Boy III is known to have been released in South Korea. However, the FM audio in Wonder Boy III will only function with a Japanese system equipped with FM audio capabilities; Western consoles modified with FM audio use PSG due to the game's region detection code. This can only be bypassed with a hardware modification or a hack in the game's code.