You can overdrive the input of the sound card. The level could be adjusted to look ok on a meter, but would not be correct. If you can capture and look at the wave file, you'd see the distortion.
Because most of the $1 USB sound cards only have a MIC input, it is much easier to overdrive them. The PCI cards often have line and mic inputs, or one input that can be switched to accept either signal range.
My solution was to use a step down and isolation transformer. I wanted isolation between the scanners and computer anyway, so rather than just use a 1:1 transformer I used a 10:1 transformer. That dropped the signals to be more in range of a mic input. It could also be done with resistors, but it was too many resistors and it look as nice as using a single transformer (or give me isolation).