I’m sure somebody where more tech expertise can give you a more technical explanation, but I do know, it won’t effect your volume levels. Most components have transformers that output a lower voltage, then with quality components there are multiple voltage regulators.
It is a little odd that your incoming voltage is changing, that much. Does the drop appear when your furnace or other electrical appliances come on?
A Power Plant would help you maintain a set voltage. For example I usually have 126v throughout the house, my P300 defaults to 117v, but I have it set on 120v, for my front end components.
In Summer is when we generally see voltage drops, when it is very hot, and people are using their air conditioning. Although mine seldom drops more than a volt or two.
I would call your power company, and ask what they think you should be getting. If you have more customers using that pole transformer, they may need to replace or add another to maintain a desired minimum. When I was younger, wall voltage used to be referred to as 110v. We would have 220v coming in, now it is 240v.
I would only be concerned if goes much below 120v.