How old is your battery? is it the original from 2008? even if a test says its good, it may not actually be "good" your electrical system is very regulated and treats the battery like a component of the entire system which is also regulated. If there is something wrong with the battery, it could cause these problems. If you bought the car used, and the battery has been replaced, whoever replaced it may not have replaced it with a correct replacement. Like in the other thread, you car will treat different battery types differently and the age of the battery. If someone dropped a new battery in there without telling the system what kind or that it was new, you can have crazy voltage spikes through the system, slowly burning out your electrical system which may or may not blow a fuse somewhere.
Every electrical pulse is monitored by your car, a dealer or reputable service shop should be able to scan your system for any error codes or funny readings. This is not the Autozone OBDII scan tool, but one designed specifically to talk to your car. You won't get a service engine light, but your car records all of its activity and if it is outside a specific parameter a good diagnostic tech should be able to identify that. Does it cost money, yes, unfortunately this is one of the things you won't be able to do on your own without investing several thousand dollars into a dealer scan tool.
Your speaker operates on a fiber optic network, like most of your creature comforts (voice, navigation, idrive, etc) and that is controlled by the MOS system which is kinda hard to break and you definitely don't want to mess with on your own.
Good luck, if you bought it used, I would have someone do a full diag, should run you about $125-$150.