> I think the article is sorta wrong about the sound setup, the Mega Drive I does have a sound input on the expansion port, and mixes it into its sound output.

The Megadrive has a stereo sound input on the cartridge port too, for a total of 3 stereo sound sources (the YM2612, the MegaCD, the cartridge) plus one mono source (the SN76489 PSG).

As far as I know no original release ever used the cartridge input, but nowadays it is supported by many flashcarts to emulate the MegaCD or to provide CD quality audio tracks (MSU-MD and MD+ romhacks), and it has been used by some recent games.