I've read that it normally goes through the multimedia speakers while there is a separate speaker for redundancy. If true, it could be that it's switching to the redundant speaker for some reason. On a potentially related note, I use Joe Mode, and when I disengage using the steering wheel in certain circumstances, the sound is MUCH louder and includes crackling/distortion. I had been assuming this was a bug where Joe Mode wasn't applied and it was happening over the multimedia speakers, but it may be the same thing you're experiencing (if what I've read is correct, you don't use Joe Mode, and the volume seems similar between the two sounds to you).