As I recall, If your Genesis 2 a/v cable does not have the full 9 pins, you will not get any audio because pin 6 will not be connected. Most genesis 2 a/v cables had only 3 pins connected Stereo left and right pins 8 and 9 and pin 4 composite video. See the pin out diagram here...
http://www.gamesx.com/avpinouts/genesisav.htm