RGB (red,blue,gren) is only video signal and can not carry audio signal.
Usually RGB aka component video is accompanied by another Red and White for stereo audio signal and those must be connected to coresponding audio line input with the component videoport.
Example picture here. http://www.avreview.co.uk/news/images/Philips_Cinema-21-9-Conn_big.jpg
Notice EXT 3 have 3 (red,green,blue) port with red and white for audio.