Have you re-located your speakers, possibly interferrence from magnetic of electrical source ?
Or uninstall and re-install you audio driver
Sometimes Windows Updates release new Audio drivers and they can mess up some Realtek or CMI Audio drivers making sound poorer. But this would have had to be manually selected by you to download these.
I would first start by uninstalling your Audio (Make sure you have your instal disk of the driver to re-install first) Then reinstall clean anfter re-boot.
Are you using a onboard sound or Sound Card.
If you have fitted a Sound Card recently then did you turn off the onboard Audio ?