The first step in determining whether it's a hardware problem would be checking the volume controls in windows (I'm assuming you're using a windows OS). Make sure the master volume and wave volume are not muted and the slider is not in the bottom position. Also check that you didn't inadvertently hit the mute hotkey. I forgot which it was exactly Fn+7, Fn+8 or Fn+9, one of those.
If that's not the case, update your sound drivers or go to your hardware profile (Right-click My Computer > Hardware tab > Device Manager) and check whether the drivers are properly installed and there are no conflicts with other devices. If you've installed newer sound drivers recently, roll-back to the old ones.
After you've checked that, and still no dice, I'd get on the phone with Asus notebook support.