I have recently installed a Sapphire HD 2600 XT 256mb graphics card into an HP tower which has an onboard Realtek HD AUDIO device. The rest of the hardware setup is standard. 

This appears to have caused a conflict which I have tried to RESOLVE with little success.
After I installed the graphics card I lost all sound.  I went into the bios and found that it had disabled the onboard audio setting.  I CHANGED this to enabled and the audio returned.

However after installing the drivers for the Sapphire graphics card I lost the audio again. I checked the bios and the onboard audio was still enabled. Strangly enough we still GOT the windowsXP startup.wav but nothing when I played a file in Windows Media Player.  We reinstalled the Graphic card drivers but that didnt work. We havent been able to reinstall the realtec drivers.Have you tried downloading the driver from the internet?If you are hearing system sounds then the sound is no longer disabled...
Check all your settings in Control Panel/Sounds and Multimedia.