Cable guide
Posted by @ 6:28 PM, EST
IGN posted an article about the PS2’s different ports and the cables that you'll need to use with them.
Here’s a bit:

The Digital Out (Optical) port on the rear of your PlayStation is the port that is used to connect your system digitally to a Dolby Digital or DTS Receiver. You will need to use a Digital Optical Cable (Also referred to a Toslink Digital Audio Cable) to connect your console to your receiver in order to get full Dolby Digital (AC3) or DTS surround sound. Digital is preferred over analog because it has a wider dynamic range, which results in clearer highs and deeper, tighter bass, without the distortion common with analog connections. If your receiver only has a Digital Coaxial input, then you won't be able to connect your PS2 to it.