PS2 Audio Through Line-In - Poor Quality

Hello,

I've got my PS2 hooked up to my PC monitor. The video quality is fine. However, for the audio, I got a RCA-to-mini (1/8" ) connector and plugged it into the line-in on my sound card, and turned on the line-in monitor. Unfortunately, the sound quality is awful. There is crackling and humming, particularly when there is lots of noise or when there is speech going on in-game.

I am wondering what would be the cheapest way to get better sound quality. I am using an onboard sound card, so if getting a better sound card would fix the problem, I can do that. But I would rather not invest in a sound card if any "line-in" setup is going to yield the same problems.

Thanks for your help!