When i plugged the speakers into soundcard line out and wintv line out into soundcard line in i got the same hissing but with a quiet ringing sound, i unplugged the line out and ringing was gone, plugged it back in and ringing was back...
Well, sorry to say, I guess we've tried almost everything possible. Got some friend with proper knowledge to help ya out in person? There's not much I could do/say left. ^^
There's another thing you could try .. plug both audio-channels from the PS2 directly into your soundcards line-in. To do so, you just have to buy (or maybe you've already got one) this little fella (img), it's about 1$. Not perfect, but you should keep it in mind if all other attempts fail .. ^^