Just got my RT2x and trying to use it with Genesis HD Retrovision cables. I can get audio fine when video is not plugged in but as soon as I plug in the green connector triggering video signal, audio drops. Trying to figure out if the problem is with the RT2x or the HDRV cable. S-video + audio is working fine from GC. Tomorrow I will dig out another console with component to see if the issue is reproduced.
Which model Genesis are you using? I use a model 1 Genesis and have noticed that I get no sound (not even mono) unless I patch a 3.5mm cord from the front headphone jack into the HD Retrovision cable. Not sure if that helps, but could be something to test.