Sam Battle aka Look Mum No Computer, a DIY musician and creator of the Furby Organ, has just released a video showing off his Sega Master System Synthesizer. In the video he takes a Sega Master System II, creates a rack-sized breakout board, and utilizes a SEGA Master System MIDI Interface device designed by Little Scale to control the sound.
As noted by Sam in the video, the console has a SN76489A PSG (programmable sound generator) chip and does not have the Yamaha YM2413 FM chip inside. (Japanese Master Systems and the Sega Mark III contained this chip).
The SN76489A sound chip has four channels: three square wave tone generators and one noise generator. While the chip inside these systems are clones of the original Texas Instruments version and built into the VDP (video display processor), they are almost functionally identical to the ones found in the Colecovision and IBM PCjr.
Sam has other video game system-related videos up including a gigantic Game Boy Mega Machine built out of 48 Game Boys, and he teased a possible 100-button Sega Mega Drive (Genesis) synthesizer in the future.
Skip to 8:42, for a demonstration of the kind of music the Sega Master System II is capable of.