If you really are trying to connect the output of the Yamaha to the input of the sound card of your PC (and not the other way around), use the headphone output of the Yamaha to the line in of the sound card, just be sure to keep the volume at a low setting on the Yamaha. If the Yamaha has a 1/4" headphone jack, you can use an adapter to downsize it to 1/8", then use a 1/8" male to male stereo cable.
Male to male http://www.amazon.com/3-5mm-Stereo-Male-Cable-ft/dp/B0002ZPJZO
be sure to get the correct length you need.
If you are trying to connect the output of the PC to the Yamaha, then use the line out of the sound card to an auxiliary input (RCA) of the Yamaha, if you want the sound on the PC to be muted, use the headphone out jack on the PC. They sell an rca to 1/8" headphone jack adapter for this purpose.
again be sure to get the length you need.