So I've got a new desktop pre-built from CyberPowerPC (Like 2 weeks ago) running Windows 8.1. And so there's been Audio crackling when I'm playing music on ITunes, Spotify, Youtube, etc. But I noticed when I would run a game (BF4, BF3, GTA IV, etc. it would stop and so I did some tests and so I started playing some music (Crackly at this point) and then started up Minecraft and watched the CPU Performance and noticed that if it went above like 12-15% it would go crystal clear but as soon as I quit MC or any other game or program it would go crackly again.
Here the Specs:
Intel i7-4820k 3.7 GHz
EVGA GTX 770 ACX 2gb
2TB SATA III 6GB/s 7200 rpm
Realtek Audio - Version 220.127.116.1183 $ Onboard 7.1 Sound
well... generally when there is buzzing, crackling or other sound distortion with onboard audio it is because of a poorly designed onboard sound module which receives interference from the rest of the internal parts.
in most cases going with a soundcard completely resolves the issue.
while i have to admit what you said (about it only happening at low cpu useage) sounds odd... i think a soundcard likely would fix the issue.
you could try contacting support... and dealing with those monkeys or perhaps getting an rma sound module (if its not directly attached to the board or doing a mobo rma if it is attached) however that isnt guaranteed to fix it.