Could be "dirty power" from aging/faulty wiring or due to the power grid in your location.
As this is purely speculation I wouldn't buy too much into the thought. If this is the case I would invest in a hearty surge protector/suppressor for either your sensitive electronics or the entire household.
There are surge protectors made for the entire household but I'm not sure how they're installed & I'm confident that they can be pretty expensive.
That's just my idea seeing as how this instance affects your router as well.
if your router and monitor are on the same power strip and wall outet it may be a bad cap in your power supply. when they get old or there an issues when they start leaking and cooking that what they sound like. to be safe i would turn the power off to the outlet and look to see if all the wires are still tight. sometimes on older homes a wire can loosen up and start arking.
i would see if you have a spare power strip and put the monitor and router on one and the pc and speakers on another. also run open hardware moniotr and watch your power supply voltages to see if there not holding.if you have an older home make a call to your power company have them check your main wire and bus box. could have been a tree limb rubbing your home wire...
if your on the 2 or thrid floor go into the attac look for chewed wires..