Exactly what kind of signal are you trying to measure? A PC sound card can only ever be used as an AC scope, mixing will allow you to measure higher frequencies, but still only as much bandwidth as the sampling rate of the soundcard allows at 44.1khz that's only about 11khz of bandwidth, pretty horrible.
If you use a variable frequency generator and mix the two signals together you'll get harmonic products but the modulation information will still be present, it's called heterodyning. So you need a variable 1-10mhz frequency generator and a mixer. Depending on what frequency you have the variable generator set to you'll scan different sections of the 10mhz spectrum. The bandwidth is so narrow though I'm not sure of what use it'll be other than a poor mans low frequency RF scanner.