Have you tested your speakers with another source such as a portable cd player?
Can you cause the crackling sound by wiggling the connectors?
Have you tried changing the audio cables?
Have you tried re-seating your sound card, or moving it to a different pci slot
farthest away from PSU It might be possible that once the load on the PSU gets
higher its interference field gets larger!
Do you have a cell phone near your speakers or the sound card? Any device
that reaches a certain frequency will cause a constant clicking noise.
What you wanna do is plug in a set of headphones and see if they
sound alright. If it's alright, then your soundcard's fine. It may be that your
soundcard is sending a signal too powerful for your speakers.
Finally: Are you 100 % sure that the Crackling Sound Coming from your Speakers or From your