How do i get the sound to work n my other dell gx270 ?

both machines have winXP home and have had the same bios, sound , graphics and chipset drives/updates applied - both show the speaker icon in the sys tray and have onboard sound enabled in bios. Essentually as far as i can tell i have treated them identically - also need to mention that when I got them they both had clean blank hard drives - yet sound works on one and not the other. Why ?