Should have started here to begin with, but...
What kind of speaker system is it, and how is it hooked up?
Can you eliminate sources like cell phones, fluorescent lighting, etc.?
Are there any changes when you try moving the speakers or connectors?
Is there another machine that you can plug the system into to determine if it is the sound system or the sound card?
Are you using a sound card or onboard audio?
And one last question...is this buzzing at a low frequency, and stays at this frequency and db?