My aux cable does the same thing in my MK6 ZS. It either crackles or only comes out of one speaker. I thought it was the cable itself but I have replaced it twice and although it seems to work ok for a while it doesn't take long for the same problem to happen again! I would love to know why it does it! Did you ever get to the bottom of it? For me it's just something I've had to learn to live with.