In Japan, Sega tried to compete with Nintendo's dominant console, the Famicom. Among the benefits its 8-bit console, the Mark III, could boast were the ability to display more colors on the screen, a somewhat richer color palette, more built-in CPU and video RAM and a faster CPU. On the minus side, the Mark III had no pause or select buttons on its controllers, a lower screen resolution and inferior audio. The Famicom's 2A03 contained five audio channels (2x pulse, triangle, noise, PCM) to the Mark III's (integrated) TI SN-76489's four audio channels (3x square, noise). Moreover, the Famicom's audio channels were substantially more versatile in hardware than the Mark IIIs.
Sega eventually released an FM Unit for the Mark III to help combat the Mark III's audio inferiority. The FM Unit contained a YM-2143 2-op FM synthesis sound chip. After it was released, most Japanese games would support both the Mark III internal audio and the FM Unit's audio. Forty-one Japanese games supported the FM Unit. Only eighty-six games were released in Japan for the Mark III. When the system was re-released in Japan as the Sega Master System, the YM-2143 chip was built into the console.
Nintendo allowed for cartridges to contain extra sound chips. which could mix their audio with the internal Famicom audio. The first product that did this was the Famicom Disk System, whose RAM adapter included an extra sound channel. Seventy-five games for the Famicom Disk System are known to support the expansion audio channel, and about 190 games were released for the add-on.
After the fad for the Disk System had died down, other Nintendo licensees who has the license to make their own cartridges included audio hardware in some of their games : Konami, Sunsoft, Jaleco, Bandai, Jaleco, Namco(t). Even Nintendo got in the act with its MMC5 chip. Twenty-six Japanese games support some form of expansion audio. However, approximately 1,055 cartridge games were released for the Famicom. Fortunately, a list of every game and the chip they use and every disk game supporting expansion audio can be found here : http://wiki.nesdev.com/w/index.php/List_of_games_with_expansion_audio
Unfortunately, neither company thought it fit to allow expansion audio in the consoles released in the West. The Nintendo Famicom sent its internal audio to the cartridge port. Games without expansion audio would simply send this right back to the console, which then went to the output circuitry. Games with expansion audio would take the audio from the Famicom, mix it with its own audio, and send the mixed signal to the Famicom's output circuitry. When the Famicom was released as the NES worldwide, Nintendo re-routed the audio so that it did not go to the cartridge port. Instead, a game could send its audio down one of the expansion pins in the middle of the NES connector, which would appear on a pin of the expansion port. Then this pin could be bridged with a resistor on an adapter that connects to the port to a pin that provided access to the NES's internal audio, allowing for mixing.
The trouble was that Nintendo never released an adapter that bridged the necessary pins on the expansion port, so the functionality went unused. Additionally, Nintendo, which manufactured virtually all the boards for the NES, never released a board with expansion audio functionality except the MMC5 boards. No other company, licensed or unlicensed, ever did during the NES's lifespan. None of the games released for the NES using an MMC5 board use its expansion audio.
Using Famicom games on a NES requires the use of a pin converter. Not all connect Famicom pin 46 (audio out) to any of the NES expansion pins. Krikzz's pin converter connects Famicom pin 46 to NES pin 54, which is what the PowerPak and Everdrive N8 NES version use.
When Sega released the Mark III overseas as the Master System, it did not include the expansion port that would allow a user to connect an FM Unit directly. In addition, the pin connector for overseas Master System cartridges is 50-pins and the Japanese (and South Korean) cartridges use 44-pins. Sega did include a card edge version of the cartridge port on the back of the original Master Systems as an expansion port, but never released a peripheral that could connect to it.
However, Sega unintentionally was kind enough to make it relatively easy to add FM sound to an overseas Master System. One resourceful hacker was able to recreate the FM unit as a board that plugs into the SMS's expansion slot : http://etim.net.au/smsfm/smsfm.html Removal of a capacitor and soldering three wires completes the job.
Doubly fortunate for SMS owners, most US/European SMS games did not eliminate their FM Sound when they were ported over. Some games were never released in Japan but still have FM sound. The only games that removed the FM sound were Ys and Phantasy Star. PS has a retranslation hack and Ys has a renaming hack to add it back in. Some other games will require a code to enable FM sound, but most will use FM sound if it is present in the system. Here are the list of games that support FM audio : http://www.smspower.org/Tags/FM
Of these games, Battle Out Run, Double Hawk, Dynamite Dux, Summer Games and Rambo III require patches to get the FM Sound working. The available patches are for the Pro Action Replay, which is a device that patches RAM locations, not ROM locations. The Pro Action Replay may be difficult to come by. In addition, Wonder Boy III: The Dragon's Trap only gives FM music if it detects a Japanese Mark III or Master System but can be patched to always output FM music. A real Time Soldiers cartridge will crash if FM sound is detected, a patched ROM will fix this issue. The rest of the non-Japanese exclusive cartridges on that list should work in an FM-modded overseas Master System.
Approximately 114 games were released for the Sega Master System in the US, and of those 46 or so games supported FM sound, most (all but five) without any modifications. That is nearly 50% of the games released for the system. Europe received about 270 SMS games, and got nine more FM supporting games, but more require patches. Still, compared to the NES, which did not receive many games that originally had expansion sound (eight cartridge games plus about eight to ten disk games), it is one thing SMS fans can boast about.