ive only had it once, a few years back,i suspected the crackle was due to overheating.A few weeks after the crackle started the Psu packed up, and also the graphic card which had a fan built onto it,the fan had also packed in.
Once the psu and graphic card had been replaced,no more static type crackle. I must stress these were the sequence of events that happened with my pc. This may have no relation to yours
Try taking the side of the case off,check fans are clear and ok,remove dust etc give it a spring clean
For the benefit of other forum members, when does this crackling start, is it when you play music from a media player such as Itunes or is it when you play media from a web site such as myspace and is your sound scheme OK in windows (Windows own sounds)Does your WMP (windows Media Player) sound OK?