I have a very strange issue with the left speaker of my imac 21,5 mid 2001: the left speaker works but the sound is low and sometimes lack of boost. Anyway, if I reset PRAM everything works fine but, after a while, the problem come back again. I also give a check to the connection internally and they are fine. At the moment I have high sierra installed. Anyway I don't think it's a OS issue since, even the starting sound is affected when the problem kick in. It's probably something having to do with EFI / PRAM. Any idea?