I have a creative soundblaster card, and a creative inspire 5.1 surround system, but the sound through the rear speakers is muffled. It comes through the front and centre speakers fine, but if I turn it up loud, the sound becomes really crackly. I have looked at every setting I can think of, but nothing seems to change it. Any Ideas?

Try downloding the latest drivers for the card. Also have you checked the speakers?, 'turning them up loud' may have blown the rear speakers.

Alot of PC speakers are really cheap quality (especially creative) and tend to give out once a bit of stress is applied to them for any period of time. I think as Darkman said that your speakers are blown.