This isn't much of a "Tweak" really but it allieviated the problem quite quickly for me, I now have no pops or crackles.  It turned out that my Hard Drive was to blame for my sound crackle/skips.  I recomend defragmenting first, if your sound still is messing up, try formating.  For me I have to replace my hard drive completely to FIX the problem completely.  Anything at 7200 RPM, with enough space for all your files will do, I personally recomend one with 8megs of cache as that seems to speed things up nicely.  Good Luck!