Great TV picture, what about sound

I've got an old ATI TV-Wonder PCI, and using Dscaler/ the picture actually looks better that going straight to the TV.

The only problem now is sound! It's in a Shuttle so it's not exactly isolated, and the card output's the sound via a headphone jack so it has to go back in via the line-in. All in all there's a very noticable hiss in the background. I've tried taking the audio out of the tv card directly into the amp, but the noise is still there.

Any ideas on on-the-fly audio cleaning? There seems to be so many video filters around...