Excellent wires brought my Sega Genesis back to normal

I believe this is do to poor wire shielding this is a shame because other wise this cable works just fine.

The wire leading out to the connection plugs is a bit flimsy, but the bulk of it is very sturdy.

But after testing the pin-outs on the socket, it seems you have the right audio channel wired to the mono signal, which would explain the muffled sound