PS2 Surround Sound?

I just purchased a Yahaha HTIB and I am having trouble getting surround sound. I connected the PS2 to the HTIB with a optical cable and I get sound fine but it isn't recognizing the Dolby 5.1. What am I doing wrong, can anyone help? Is there any good web resources for noobs like myself to get pointed in the right direction? Thanks!!!

To answer your questions:

I did go into the PS2 menu and make sure it had recognized the optical output.

The game I was trying out was GTA:San Andreas. It recognizes PLII and only uses the Front L&R. What confuses me is that in the game you can change the audio to Dolby 5.1. Why would they have this option if you really can't use the surround sound?

Thanks for the help, guys!