if you have updated your sound driver,open your sound manager & check everything else on the settings that corresponds to your speaker channel
(4.1 if am not mistaken).
then try the speaker test if all of your speakers are firing up
(front,rear,sub).
check your input/output connections (mini jack,RCA from pc to speakers), make sure they're all plugged properly & no loose connections into the sockets.
if you have a motorized equipment running around,then try plugging your speaker (amp) to another A/C wall socket.
you can try your speakers to another pc or try borrowing a speaker to test & see if either you have a bad sound card or your speaker's built-in power amp is causing the problem.