Well typically,
White is left channel audio.
Red is right channel audio.
Yellow is composite video.
The red and white rca's will provide audio to your receiver/amplifier. The yellow rca provides video signal, which is not a high resolution format(composite), limited to only 480p. The video connection is not needed to get sound to your theater audio. If your using your pc to feed your home theater rig, use a 1/8" to rca adapter to handle the sound. For video, you want to use svga(15 pin d-sub) or hdmi connection to achive 1080p resolution.
hope this makes sense.