I use Sony Vegas 9 Platinum Pro and every audio song/clip I import crackles or randomly skips when put into the timeline. Even my audio form my video camera, cds, etc. This makes it very hard to edit.
Without seeing the info requested, I would guess this sounds like an under spec system for the work it is doing.
Every time I have had crackling, its due to an overloaded system (CPU, ram and disk).
Skipping is an even more overloaded system.
If the project renders without the crackling and skiping, then its definately a lack of grunt. Fast PCs with fast disks and maxed out ram can fix these issues.