Buzzing sound developed on speaker system twice, any ideas as to cause

On the ground floor of our townhouse, I have my computer workstation that includes of a large CPU off to the side and two 22" Samsung LCD displays in a semi-circle arrangement. Behind the displays, I have two speakers. These then are flush against an exterior wall. There is a light switch near the speakers of this exterior wall, and a circuit breaker box to the far right (about 3 feet away) from the CPU and monitors.

About three months ago, I noticed the right speaker developed a persistent hum when turned on. Moving its location did not increase or decrease the hum, it remain constant. Part of me wondered if EM interference had altered the speaker, but part of me also wondered if it had just gone bad.

So I replaced the speakers with a different set of speakers. At first they worked fine, but now, three months later, the right speaker has yet again developed a buzzing hum when turned on. The left speaker does not have the hum.

The fact that two different types of speakers both developed persistent humming on the right speaker now makes me seriously concerned about EM interference. Any ideas as to cause(s) and resolution?

Could the Samsung LCD panels cause it? If so, any fixes?
Could the back wall with its light switch and circuit breaker cause it?
Could something be wired wrong within the wall, and if so, how to test and how to fix?

When we first moved in, the circuit breaker had a slight hum associated with the circuit break for the downstairs (associated with the computer) but that went away after a month. But maybe that's connected to this problem?

Many thanks for any advice!