i don't have a PS2, but i can still say that i don't think its a problem with your reciever accepting a digital signal. if you are hearing the game then the signal is being sent correctly to the reciever. the problem you seem to be describing is that its just not playing in the sound format that you want.
i would look for these things:
1) my reciever has a "listening mode" button that switches between stereo, pro-logic, mono, etc. this mode is set differently for each input. for instance my vcr input is set to pro-logic and my game input is usually set to stereo. check for this on your reciever and see if that's the problem.
2) i'm not positive but doesn't the PS2 have a sound option? if so then check to see if this is set correctly.
again, if you are hearing the games then the reciever is getting the digital signal. you just have to find the right settings to get the sound you want.