Thank you so much for your detailed answer! I learnt a lot from it.
I did another test.
I recorded sound from my radio through Line In. The playback quality is just good in stero or mono. I think there should be no difference whether I record from TV Wonder or my radio since they both are from analogue sound source.
So, the problem maybe lies in the video capture program itself, I guess.
My another question is about the Closed Caption. I can see the closed caption when view alive but never see it when playback or time shifting.
ATI vcr file should support this.
Thanks again for your help!