I've got the same problem with a slightly different setup. I bought a new video card (no external power source needed - and it has an integrated sound card, which I disabled through Control Panel) and I use an external USB audio card (Line6).
The sound crackles when I watch a video (or when I'm clicking constantly on a drop-down value), but doesn't if I play a game with all the graphical features...
I have an older motherboard with an on-board audio card which I don't use (but I haven't disabled).
I have reinstalled the drivers for the line6 soundcard, but the problem persists.
Edited by hamluis, 17 December 2011 - 01:56 PM.
Split from different topic, PM sent new OP.