Bluetooth: A2DP stopped working
Finally I found the solution: When in car mode (automatically toggled using Llama when my car connects via Bluetooth), I use Car Dock Home v3 as home replacement. Inside it's preferences in the "Car Mode System Settings" there's an option called "Samsung USB Mount Au..." (can't read the complete entry). I had already forgotten that I checked this option since I wanted to know what it does. Now I know that it mutes A2DP ;) No Idea what it should do but fortunately now I have sound again.