generating a 10mVpp signal from computer
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Audio attenuator. This gives a signal output from zero to 1% of the input, depending on wiper position.

The circuit above is what you require. Make sure you identify the wiper correctly. It's normally the centre pin of three.


  I did some quick research and it seems most built in audio cards have about 46 Ohm impedance.


That's fine. This circuit won't load that to any significant extent.