In my setup official one did it too.(I know I wrote it before :))
Nintendo Scart cable is using 1 ground for both video+audio. Normally in a Scart spec, r+g+b lines have their own ground pins, same for audio l+r.
So I think if you want the best sound from that console wire the audio signals and their ground separately and mount 2 RCA jacks, and use a high quality cable for it.