Try plugging everything into the same wall socket, that should help.

When you run stuff off different wall sockets you create a ground loop which acts as a big antenna. Any radio signals floating about that are at a frequency the big old antenna can pick up...are. These are then amplified by transistor amp type circuitry which then comes out the speaker. Just like a big transistor radio.

As you can tell, I'm not totally clear on the exact mechanics of this my self however I bet LATS could put you rite !