For the most part, audio electronics should be able to operate within a +/-10% range. A range from 115 to 125 volts is just fine because pretty much all audio electronics have some sort of voltage regulation when converting this to DC. That being said, it could definitely affect the sound quality if it's widely outside of this range. Anything much under 115V will have a very weak audio output (no impact/attack/muscle). Anything over 125V could push too much of the "solid state/harsh/sterile" character.
I'm not sure what was happening in that example thread where the turntable was failing -- maybe the turntable doesn't have voltage regulation and is just running off a straight transformer from the A/C? Turntables are different beasts and I don't have the experience to comment. I do know that the speed operation on certain turntables are critically based on the 60Hz waveform coming in from the A/C (if it's an A/C motor instead of a D/C motor).
Isolation transformers could have winding errors. You could definitely get a transformer what was increasing or decreasing the output voltage. They are auto-wound and it's a random outcome. They usually do not measure the voltage gain output during manufacturing because of cost issues.
