Trying measuring the temperature by pointing the
View attachment 62461, or something like that but make sure it is a good quality one (not something listed for under $15, just an example...) at the same spot when the thing sounds good, and not as good, to you.
It would be normal/possible what you perceived was real, if the temperature was much higher when it doesn't sound as good. Any subtle difference would likely not be real because of "bias".
As to what Paul McGowan said, it is hard to say if that was real, valid, bs, or something in between. If you have a multimeter, you can also measure the voltage in the morning, afternoon, evening etc., to see how much it fluctuates, chances are it won't fluctuate more than 5% any time of the day, otherwise you should complain to your electricity supplier.