check a couple of simple things:
is there a headphone plugged in? -> remove it
is the volume icon present near the clock? if yes, check all the volumes. if not -> reinstall sound driver from motherboard cd or internet (look for your motherboard's model, you'll find it)
check every connection on your speakers: if it is a surround (like mine) check the back of the subwoofer, or the back of the pc.
if your sound driver is properly installed, open the program (usually near the clock, or hidden, via HD, etc sound driver): double-click it and check the configuration (stereo, 2.1, 5.1, 7.1,) then run a sound test.
and last (this may be the most trivial, but hey! it happens): check if winamp, vlc, or any other media player is muted. silly? sometimes overlooked :)
if none works, restart computer, uninstall driver and download the latest from the internet.
have fun!