At a very crude level, look at a dumb electric clock. Does it keep time? Note that some clocks will still keep time at 90V, but the control system may drop out.

A bad connection can cause the voltage to sag significantly when the power line is under heavy load.

Take a voltmeter to the site and measure every witch way while heavy loads are operating. (air conditioning, furnace, refrigerator, and such) There could be a problem in the branch circuit that is powering your system.

I don't know what type of voltmeter you are using. Some voltmeters include a logging function. You can also set up a latching relay circuit, latch the circuit and monitor it with a remote camera. If the light goes out, there was a drop on that circuit.

If line voltage is actually sagging, the likelihood of fire increases.

Edit: Will other units report their up time? For example, if there is a Sonos system, you can submit a diagnostic and ask Sonos support when each unit was booted. This could be handy information if the players are scattered around the house.