I've been playing it for a few days and it was working fine, but now when I load the game, I sometimes have sound and sometimes don't. More often than not, I don't have the sound.
I have updated all my drivers. I have uninstalled and reinstalled it.
I know my computer can run it, since it does work fine sometimes.
Does anyone have any idea why this is happening?
Thanks!

If you have XP, try reducing audio acceleration to one notch above none.

Here's one way to do it
http://support.frictionalgames.com/entry/19/

You can also reduce audio acceleration through the Sound tab on dxdiag.

Have you tried compatibility mode? That sometimes works in games.

Is Nibiru the only game you have a sound issue with ?