I think you have to buy a special A/V adapter, and then an optical cable, go to bestbuy.com and just search for playstation and cables and you should find what your looking for...
better yet, go to BestBuy and figure out exactly what the adapter cable(s) you need looks like, is named, and is made by whom...then go home and look it up on eBay where it's probably selling for about 1/4th of Best Buy's price.
Electrical wiring, cables, adapters etc. for audio, video, cell phones, computers, etc. are usually jacked up obscenely at Best Buy and CC. The same RCA audio cable that Walmart sells for $5 can cost $15 at BB/CC.