PC Speakers (bum bum or mmmmnnn sound)

[Meian]

This is probably an obvious answer to this question to the speaker savvy people but...

Several pc speakers throughout the house and my father's office (they are different kinds except for two sets of them) after about a year or two start to make a mmmmnnnnn sound from the main speaker when there is no sound on or the pc is off.

Also, there is a bum bum bum bum bum bum bum bum sound on some of the older speakers when they go on (at one point on one set of speakers, the bum bum bum sound was occuring even when the pc was off)

All these instances are occuring most often with the speakers and pc off (but speakers still plugged in)

How do I prevent this or stop it.

[Kaisha]

I only find that happens when the master volume on the speakers themselves are set too high, or that it's a speaker set with cheap wire insulation, and it's receiving interference from outside sources.
(my brother had a pair of speakers that apparently picked up and played a particular radio station when shut off...)