That's a curious problem. My first thought is: have you somehow ended up muting your entire Mac, or are you hearing audio from other applications?
You can check your sound settings by going to Apple --> System Preferences --> Sound, where you'll see this:
Note both the bottom volume slider and the mute checkbox to ensure they're set properly.
The other possibility is that you've somehow deselected all the Entourage audio notification sounds. You can check that possibility by going into Entourage and selecting Entourage --> Preferences --> Notification:
Those should cover the main reasons you'd no longer have any notification.
Hope that gets you back up and running!
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I have had the same problem since I downloaded an Office:mac 2004 update. Other apps like Yahoo IM still produce sounds but Entourage remains mute.