Yep, they buzz when pluged into the outlet and not the computer.
The plug-in runs from the outlet to the sub, and then from the sub to the speakers. The only thing that makes the buzzing sound stop is to unplug the cord that runs from the outlet to the sub, unplugging the speakers or laptop from the sub makes no difference.
Now here's an interesting fact: I used to plug my laptop into my crappy $100 shelf stereo system using a mic to left/righ audio connector (I don't know what that's called) and after about a month the same buzzing/humming sound happened to it, and even when it wasn't plugged into the laptop and just playing the radio or a CD I got the bad sound. The stereo was about a 2 years old, and like I said, really cheap, so I thought it was just crap and it's setting in a closet. This may have nothing to do with anything, but I thought I'd mention it, as it is a strange coincidence. Is it possible that my sound card is messing up my speakers???