Ok, so in other words onboard sound...
Few things to check and troubleshoot, most are pretty obvious.
Speakers
-try plugging into another power source, socket etc
-check the cabels are ok
-plug it into something else say your IPOD etc, and see if the buzzing still there, then plug it into a hi fi and test like before
-try different pair of speakers and/or headphones on the pc
-confirm that it's plugged into the correct jack, irrespective if you have jack-sensing enabled on your pc
-unplugg those speakers and see if the sound's still there
PC
-Observe if that sound only comes on when you move something, play something.. basicaly, look for anything that can be causing the sound to come on cue (like when you click your mouse on a link)
-double check all settings correct
-disable jack sensing on your audio driver (if it uses it) and confirm your speakers are connected to the correct plug
If all else fails, and we can't determine whats what, see if you can record it somehow and upload it to a free webserver/host and give me the link.. encode it to .mp3 and zip it..