Please note that the problem we're pointing out here is *not* an output volume setting problem. The very first time an newly-launched instance of Entourage 2008 tries to play a notification sound based on an action, it will play just fine. (That might be the welcome chime. I have this chime turned off, so the first sound played on my computer might be a result of no-mail-fetched.) After the first sound is played, no sound. Before the first notification sound based on an action, the speaker icons in the Preferences window will play the notification sounds. After the first notification sound based on an action, the speaker icons in the Preferences window *will not* play the notification sounds.
I've ruled out the output volume setting problem. I've also used Audio Midi Setup to rule out a sampling rate problem.
(So turn off your welcome notification sound, then relaunch Entourage 2008. Choose Entourage > Preference > Notifications. You'll be able to play all of the sounds by clicking the speaker icons. Once an action triggers one of these sounds, no sounds will ever be played again. So...the sounds are there and you can hear them...until the first sound is triggered.)