There is a system wide setting for notifications that you need to check first.
MENU-SETTINGS-SOUND.
In this menu are a few things you need to look at. Click on VOLUME and make sure the sliders are correct (and that your notification one is not all the way down).
The go into NOTIFICATION RINGTONE.
Pick one. Any one. Make sure it makes the sound back when you select it. This is the system wide notification setting- default notification setting.
If this works, all your notifications should make the above sound. If this works, each application you can change the default ringtone to a custom one just for that app (Handcent, Talk, Gmail, etc). I have had the default ringtone get "stuck" before, and changing the system wide ringtone did the trick. Now that I think on it, I think what I actually did was use RingDroid to make a notification sound, and when it asked if I wanted to make it my default, I said YES. That seemed to free up whatever was stuck in my case.