Under these conditions, some methodical trouble-shooting will turn the trick. Audacity is a deceptively large and versatile software package, and requires some perseverance to tame the beast.
(Jimi Pocius does most all his recording/production/mastering with Audacity - http://www.songplanet.com/artists/bands/1172/ - and I have done complete projects with it also. Very versatile and reliable.)

First, import a stereo audio file you know to be "crackle-free" into Audacity, and play it back through each audio card. Sound OK?

If Audacity's output sounds good with imported audio: Make certain Audacity is set to the default sample-rate of your Audio card. Record something simple (at an input level that is well below 0dB), and then apply the "normalize" function to the resulting audio set to -0.1 dB (to make absolutely sure your levels are not clipping), and play back.

Let me know how it goes.


just looking for clues...
Oren.
http://www.masteringmatters.com