Ok I have redone this computer and for some reason I cant get any sound. I have onboard sound on my ASUS A8N-SLI Deluxe 939 NVIDIA nForce4 SLI ATX AMD Motherboard. It has worked in the past. I installed all the drivers from the cd that it came with. I believe it used the Realtek AC '97. I have my speakers plugged in the right slot - the green one but it wont work. Any help would be much appreciative.

What do you mean 'redone'?

well i replaced the HD, vid card, case.

Wat video card?

actually its still the original video card i had with it, i replaced the case, HD, PSU
 
EVGA 256-P2-N615-TX GeForce 7600GT 256MB 128-bit GDDR3 PCI Express x16 SLI Supported Video Card

Do you have front panel audio on your new case?

what jtt283 said..  front panel audio plug.  either defective, not connected, or incorrectly wired.
 
if there is no front panel audio in the case, then you may need to put a pair of jumpers on the front panel connector on the mobo.. consult your manual for that info

ok can i not play it through the audio connectors on the back?

You didn't mention the OS, Vista SP1 caused some major problems with sound card drivers.  
 
I have a different mobo but I had no device conflict and had no sound for a while after SP1.  It has since been fixed but it was frustrating at the time

As another mentioned, you don't say what OS your using.  Since you replaced the Hard Drive, is it correct to assume that you reinstalled the operating system and NOT cloned your old drive to the new drive???  
 
Go into Control Panel - System - Device Manager, then Sound, Video and game controllers.  Check if your sound card drivers are installed.  You can delete the device from there and have windows try to reinstall the drivers.
 
Goto the ASUS website and download the most current audio drivers for that MB.  The original drivers obviously worked, but you might as well use the latested.
 
As for the jacks, normally the light green audio jack is the one to use for 2 channel stereo speakers.  Using the rear jacks on the MB is normally the ones to use unless you want extra wires hanging off the front of your computer.
 
Finally, are you sure you have the sound turned up in windows??? The speakers powered up and volume turned up???  Can you test the speakers on another computer???  Remember, check the easy stuff first.  Won't be the first time something simple like the wires going to the speakers going bad......

Ok i have XP Sp2 and i have tried the speakers on a different computer and they did work. Everything seems to be installed correctly - all the drivers - no yellow exclamation marks.