Many av receivers with zone 2 outputs will only output analog sources. I suspect that is your problem. You could use the analog PC output or if you want the digital surround for your PC then you could split the optical signal, feed one leg to the Onkyo, the other to a digital to analog converter, and that to the Onkyo using another input that is analog. If you are trying to send Dolby Digital to the 2nd zone you will need a dac capable of accepting this type of signal (most only take PCM).