I can't find a place where this mod is explained...
The MD have so many excellent melodys that it looks really useful do try this mod.
Can somebody find some shematics or explainations ?
There aren't any, as it's not possible in the same way it is with the SNES.
Basically, the SNES has a single package that handles generating the sound in digital form. It then passes that digital signal to a digital to analog converter (DAC). The signal to the DAC can be converted to SPDIF via a converter chip.
With the Megadrive/Genesis, the sound is generated by two different chips that only output an analog signal. As there's no digital representation of the sound, there's no way to generate an SPDIF signal easily. In fact, the only way I know of is to take the analog output and then convert it back to digital.