Hearkening back to my days of being an engineer in a radio station...
A large fraction of audio buzz in sound systems is caused by what's called "ground loop." The theory of grounding is that by connecting compnents to the ground, you establish a common "zero" across all components -- if you test across any two points on any
of your ground cables, you should get zero voltage.
It's a nice theory, but in practice since there is resistance in all wires (and in the ground), there's always some amount of voltage difference through the grounding system, and if alternating current leaks into it, you get a buzz. The fact that the buzz you're
hearing is in the 50-60 Hz range is a very good indicator that you have a ground loop problem.
If it is indeed a ground loop problem, that would require:
1. two electrical devices, both grounded (e.g. plugged into the house mains by means of a 3-prong plug). You could get this by having a PC and separately powered speakers.
2. The electrical devices are interconnected (e.g. your poswered speakers are plugged into the audio card in your PC).
3. something is leaking AC current into the ground wire. This could be the power supply from either piece of equipment, another faulty component inside the PC, or (unlikely) even another electrical device nearly plugged into the mains (like your fridge, or
Ground loops are notoriously difficult to isolate. So here are some things to try:
1. Disconnect your speakers from the PC (and from the mains, if powered) and try listening to your PC audio through a pair of headphones. If the buzz is gone, you just found your loop. That doesn't mean that the speakers are necessarily the problem, it just
means that they are part of the loop.
2. Disconnect every non-essential peripheral from your PC, including your printer and particularly everything that has separate power.
3. leave your computer going with audio playing (so you can hear the buzz) then walk around and unplug appliances one at a time until the buzz goes away.
4. If desperation is setting in, go to a hardware store and buy an adapter that turns your 3-prong plug into a 2-prong (groundless) plug. Try using it BRIEFLY on your computer to see if it makes the buzz go away. Be aware that it is potentially dangerous to
run your computer without proper grounding, so you do this at your own risk and this is NOT a long-term solution -- just a diagnostic tool.
If none of this works, post something here again and we cain brainstorm some other ideas.