RGB cables, red/green/blue, are component video and are ALL video.
Yellow/white/red cables, composite, are yellow for video and red/white for two channel audio.
If you have the yellow/white/red configuration, you will run the yellow cable to the PJ composite input and the white/red will need an rca to stereo mini-pin that you will run to the HTPC aux input to play the audio over the HTPC speakers.
// Front Projection Enthusiast //
BenQ W1070 - 103" EasyFlex Painted Screen
Onkyo TX-NR646 - JBL Northridge 5.1 - SVS PB-1000
Roku 3 - Amazon FireTv - Sony BDPS5500 - Kangaroo PC