Since they don't buzz with the PC, I'm guessing you have a ground loop problem. To check that you could run an extension cord to another wall outlet and plug your speakers into that extension cord. Or just buy a 2 prong plug adapter and don't use the green ground wire( or buy one without the green wire).
You can pick one up cheap at the store and it will cost a lot less then getting new speakers.
Happy Listening, the Prisoner.