Hi luxyan, please enter the bios & make sure you're SoundMAX Audio is set to Enable. (To get the bios to load, press the Delete key every 2 seconds untill the bios menu opends).
Save & exit the bios. Let windows finish loading.
(Windows may ask for the sound driver software). If not than open control panel than open add/remove programs & uninstall any driver software for you're sound card, if listed. Next open the System icon. Click the hardware tab than click the device manager button. Scroll down to Sound, video and game controller. Find the yellow ! point.
Right click once on the ! point & than left click update driver in the pop-up window. Windows Hardware Wizard will open, click No, not at this time & click next. In the next window click, Install from a list or specific location [Advanced]. Than click next.
In this next window click on the green check mark next to, Search removable media [floppy,CD-ROM...].
By doing this the green check mark should go away. Click the Browse button & find the folder where you downloaded the sound driver software. [from Asus oficcial site]!
Click to open the folder & point to (highlight) the folder that holds your driver software. Watch the OK button, it will slightly highlite it's self (meaning windows will search that folder & install the sound driver software if found). Click OK.
Re-start you're PC. Try you're speakers. If you still have no sound than you're onboard sound chip is defective. You're board was made back in 2003, Asus warrants their boards for three years. You're only other option is to buy a PCI sound card an add that to you're PC. (ASUS P4S800D-X link).
Link to some PCI sound cards.
Best of luck to you!