I too have had the same problem on my system, so this is what worked for me and what I would suggest:

Open your Volume Control.  To do this go to Start, Then Run then type mmsys.cpl then hit ok (alternatively you could go to your control panel then select Sounds and Audio Devices).  To get to the volume control you will need to click on the Advanced button located under the Device Control (see picture below, i have circled it in red):



once you click on Advanced, you will be in the Volume Control, so you should see this:


Now systematically go through and one by one, set them to Mute.  If the buzzing goes away, then you can eiter leave that one on mute or unmute it and turn it down.   For my system the "SW Synth" causese buzzing, so I have it turned way down. 

If for some reason you can't see all of these settings, then, while in your volume control go to File->Properties, and then put a check mark next to all of the boxes listed, like this:



PS.  Welcome to the FixMyXP forum...

My speakers often make a buzzing sound (mains hum) and that can be when they switched are off!

Hum in speakers can be caused by any interference and also due to the power supply to transformers. It could be due to the number of devices that are connected to the same power socket, other devices and appliances that are nearby, mobile/cordless devices or simply a power supply that isn't 'clean'.

You might be able to identify what the cause is by elimination of other devices and appliances. You could try shielding all the leads to the speakers with tin foil.  You have eliminated the sound card and the speakers themselves as being the problem. You have also eliminated a software problem.

Or you could do what I did - ignore it.