Because RGB=Red, Green, Blue chroma signal=no audio. Are you tryng to use the TV for sound? If so you need to use a separate cable from the PC audio out (usually a stereo mini) to one of the TV audio inputs (in this case I bet there is a pair of RCA type inputs associated with the RGB section). If this is true you will need the cable to be stereo mini plug to RCA left/right plugs.
Those cables cannot carry an audio signal. You will have to use an HDMI cable if you wish to output sound to your tv.
Right, you either have to use HDMI on the PC/TV sides, or use component (RGB) and another cable from your soundcard/on-board sound to the TV's component connections. For the last option you'll need a cable that goes from 1/8" stereo to 1/4" RCA (L and R channel).