I had a similar problem when I first got my first 6950. I didn't use the bundled software, I just downloaded the CCC update. It was throttling down my core from 880 to 600.
HOWEVER
The crackling you talk of with your sound card indicates "dirty" power.
I would check again that:
1: all firmware + drivers are up to date and/or the most stable version.
2: All clock settings are correct and stock.
3: You are using a proper PSU power cable. Using a thinner one can lead to problems.
If all of this turns up golden then either you have some mega-gremlin in your machine or your PSU is old and is giving you dirty power. "Dirty" power refers to, in a DC system, having the voltage vary. This is due to old components in the PSU and/or bad power at the plug. I have this problem with my audio equipment at my studio, but audio equipment and a sound card are two very very different things.
Good luck.
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