DVI does not actually carry a sound signal. There are several solutions you can use based on how your TV's DVI input operates:
1. If your DVI input is tied to a "PC input" there's probably a miniplug input for sound on the rear or your set (for running a mini plug from your pc/laptop.) In this sort of situation, this input is usually your only source of audio input for the DVI signal, so you'll need to leave your RCA cables plugged into your PS3, and run your Red and White audio leads into an RCA to miniplug adapter. Then you plug the mini into the headphone jack-looking audio input on the back of the set.
2. Your display may have an RCA input set for the same input as your DVI (Video 2 for example). Again, leave your PS3's RCA cables plugged in, and run your red and white leads directly to the correesponding jacks on your display (If your DVI is 'Video 4', look for audio inputs labelled 'Video 4' as well).
3. If you have a stereo system hooked up with an optical input, just run an optical cable from your PS3 to the stereo - super easy. Some TV's may also have an optical passthrough, where the set receives the optical cable and passes it along to the Stereo receiver through either another optical cable or HDMI.
IMPORTANT to remember: Make sure your PS3's audio settings are set to the correct output. You'll find Audio settings under the 'settings' options in the XMB. This is something that is commonly overlooked.
Best of luck, and hope that helps!
As the ancient Tibetan philosophy states:"Don't start none... won't be none...".