A lot of televisions share a common input for the RCA connections. One set of RCA's uses the yellow for video, and red and white cables for sound. The other set is called component and uses 5 RCA plugs, red, blue, green for video and a second red and white for sound. In the settings there should be an option to pick between the three plug yellow, red and white cable or the 5 plug red, blue, green, red and white component cable for the RCA input connections. Your TV input option for the RCA cable is probably set to component. That is why you are getting sound with the yellow, red and white cable and no picture.