Maybe, but unless I'm mistaken when you record audio direct from the PC it isn't actually going through the sound-card/on-board audio at all... The job of those components is to convert the digital sound signal from the PC to an analogue signal to be sent to an analogue device (speakers/headphones)... But when you're recording game footage all you're doing is capturing the digital sound signal; no need for it to pass through the sound-card at all
When you're also recording a mic you are relying on the sound-card (but usually if it's a USB mic it's built into the mic itself) to do the opposite conversion for the mic signal (analogue -> digital) at which point the quality of that processing becomes important
This makes your crackling a little bit of a mystery - I wonder if it could actually be an artefact of the rendering somehow?
Thanks for the infomation, I'll keep an eye (or ear) on it and experiment with settings.