SWMBO's radio is possessed. While I find it hilarious that she comes back to her car blasting Biggie in a busy parking lot, she is less than thrilled. She has a 2005 e46 325 and the radio will occasionally turn itself on or off and blast the volume. Is this maybe the automatic volume control for speed going haywire? Diagnosing electrics is out of my echelon. Any ideas? Didn't see anything on the googles about this
My Land Rover Discovery did that once when the battery was low on charge. I got soft rock at high volume-I WANNA KNOW WHAT LOVE IS..........I WANT YOU TO SHOW ME! 
The most wacky things happen electrically when the battery begins to fail.
The dash gauges are all ok?
It's cheap enough to test the battery and at least exclude it from further diagnostics.
If you can access the schematics somewhere, I would start by finding the ground for the radio circuit. Inspect and clean it. Grounds are the number 1 cause of crazy electrical issues in my experiences.
Number 2 cause on crazy radios that I have seen is coins inside them. Do you have kids or do kids ride in the car? They LOVE to stick money in radios. Happens all the time and once you have conductive material bouncing around on the boards weird things happen. Pull the radio and flip it upside down. Listen for the jingle that means "not warranty".
I say with certainty on that model but if you are still having issues adter all that you are most likely looking at a head unit.
I will sometime turn coworkers volumes or heat/ac up if they leave the doors unlock in the parking lot. Doesnt work on newer vehicles. 
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