Are they serious? How does a 3rd? gen system only have one audio output, do you really need to mod the system for stereo?
In Japan, it was the Megadrive systems that usually came with mono cables and you had to use the earphone jack(when there was one) to get stereo.
Sound has been one of the PCE's feature's since early on(AV/Turbo Booster, CD-ROM's, ROM ROM Amp, Fighting Cushion) and everything after the original white version has had stereo out from the get go.