F4/F5 no Sounds?

I have a powerbook G4 and just recently the popping bubble sounds that get louder and quieter when you press F5 and F4 aren't making the sounds anymore. But they still change the level of the sound change, they just don't make the noises anymore. Any ideas how I can get the sounds to come back? Thanks.

system preferences
sound
play feedback when volume is changed
[check it]

checked...still not working...

does sound in general work
?

Yes it does, and it goes up and down with the buttons... just no popping sound.