Here's another long shot: Check your CPU temperature (or graphics card temp). I have to blast dust out of my CPU fan every several months or the fan bogs down. The CPU starts runing too hot in a demanding app like Cake, and audio starts to crackle. Audio is generally fine for less intense tasks like streaming video, etc. The crackling also gets gradually worse over time. So it may not be your problem, but it's worth looking into, because it took me a while to figure that one out. Some locations are dustier than others.