No sound with mini to rca?

i am using a VGA cable to get picture on my philips 42 inch LCD tv, and a mini to rca cable for sound. the picture is fine but i dont get any sound, i tried to switch my playback setting to SPDIF out but that didnt work either. I want to use DIVX to watch movies, i am running windows vista and have a Gateway laptop. What can i do to make this work?