This is on the BMW, but the problem is likely no different regardless of make.

The car has an eight speaker stereo, and all eight used to work just fine. However, one day out of nowhere I noticed I had to turn the volume way higher. I couldn't notice the problem at first because the speaker that remained working fine was the driver's side rear. When a passenger rode they said they could definitely tell that the only speaker that seemed to be working was the driver rear. I then tuned the balance all the way to the passenger side and there was just a little sound coming from all of those speakers, and the same is true with the front driver's speakers.

I don't know what this is that is causing it, but all speakers do work, only one of them works at the proper volume it should. Ideas of what it could be/what to check?