various companies make a device for cable boxes, this is most times the biggest culprit. To test this just remove the cable to see if most or all of the buzz flys away.
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A couple of fairly cheap ways to lower the voltage.
Check out this on ebay,
Another way is with a buck/boost transformer wired in a buck configuration.
What is the voltage rating of the audio equipment, 110V, 115V?
I assume when you are taking the voltage readings you are taking the reading with the load connected, the equipment turned on.
What time of the day is the voltage 120V? What time of the day is it 125V?
The next time the voltages is 125 volts take a line to line voltage reading. See if it is 250 volts. Is it less than 250V? How much less?