Speaker Crackle

If you're using a creative card, dump it, their crackling issues are a nightmare.
Your card could be dying, but I doubt it.

I hear that some video cards can cause heavy interference if you OC or installed a faulty one.
Your dpc latency could have been increased by some conflicting driver or device or a bad bios version which might cause that, check yours: DPC Latency Checker

Digital out would probably fix your problem depending on the card, but DACs aren't cheap.

Jam it back in, in the dark.

If your dpc latency is spiking up a lot instead of a constant fairly close value, it's either some driver, device or your BIOS.
If you have a gigabyte motherboard, they have a ton of issues with dpc latency, you'd have to google to figure out what bios versions have it working right..

When I was using an audigy 2 the KX drivers helped with the crackling, but it came back later and never went away after that, assuming it's probably shot now. Don't think you can use them for an x-fi though.
Only other thing I can think of is manually setting the latency for the card (this has nothing to do with dpc latency afaik)
Guide to making your x-fi not crackle
PCI Latency Tool 3.1 v2
Nothing helped with my audigy, though, tried everything.

There's nowhere I can't reach.