First: WELCOME !!!

You say, that the DVD Sound comes from your TV ??? Just to be sure: You have the Optical Cable (OC) connected to your Receiver/Amp ?! Right ?
You have to switch the PS2 to Optical Out in the menue, than you have to go into the DVD settings while playback a DVD (Select) and switch Dolby Digital and dts to ON (maybe you´ve set them to OFF ?!)
At your Receiver you must switch to the right INPUT, but you must have it already, if you hear the Gamesound…
But I still don´t understand how your TV is able to playback the DVD Sound if there is only a PS2 – Receiver Soundconnection ???!?!? Just plugoff the soundconnection to your TV, maybe your Receiver get´s “irritated”… Just a guess…
Last chance is to unplug everything and just plug in the OC into the PS2 and the Receiver and listen what happens !
Tell us if it works or not !!!