I had this problem one day - I have a 2 hour trip to work and it really bugged me. Same issue, balance set ok, just one side of the speakers went one morning. I put up with it for a while on the radio (about an hour) and just before I was about to call the dealer, I changed to a CD and the problem wasn't there. Went back to the radio and no speakers on one side - so I knew it wasn't a bad connection. Changed back to CD and after 30 minutes tried the radio before calling the dealer (assumed it was an idrive problem) and everything was magically ok again. Only ever happened once...