[sonick]

Another day, another random electronic gremlin on a BMW. This is on my premium audio system with BMW professional head unit on my 2009 e82. Should be very similar to e90x systems.

Symptoms/troubleshooting findings so far:
- no audio from radio, aux, CD, etc.
- no door chimes
- no SOS malfunction error code
- MOST optical loop is intact (the diagnostic port under driver footwell is lit)
- when I turn up the head unit volume to max, I can hear audio from the radio and door chimes very faintly, which leads me to believe the audio signal is intact just not amplified.

I checked connectors going to amp and out of the head unit, no issues there.

I am leaning towards an issue of the amplifier since I can hear the correct sounds when I crank up the head unit. Also because it affects the door chime as well, which would not be the case (I think) if it was a head unit issue.

I took it out and opened it up, didn't see any obvious signs of damage on the circuitry.

Another possibility I've read is that the battery is low and so it's going into a power conservation mode, but I don't know how to confirm this. I have noticed in the past year I would get battery warnings when I would leave the car on too long without the engine running, but have not experienced it since this sound issue.

Anybody know how I could diagnose the amp or battery short of buying a new one to try? Or if anybody has come across this problem?