Have you checked that the speakers are working..? Plug them into an iPod or an MP3 player and see if you get sound..Also try plugging a set of headphones into the computer and see if you get any sound...Are you using connection at the back of the computer or connection on the front of the computer...Look for the Volume Icon on the tasbar and see if you have the volume turned up...Normally the first thing to check.
Are you using the onboard sound on the motherboard or a soundcard that you plug in???
Infact can you list the motherboard if possible, soundcard if you have one, what make the speakers and sub-woofer are, whether its 2.1 , 5.1, or 7.1 cheers.