I just figured mine out!... so I recently installed an app to boost my speaker volume...ya... iclarify or something.. anyways I uninstalled it and did a soft reset..(home and off button tell you see apple logo) then all my sounds came back! COOL..
So try remembering when you last had sound.. and what the least app you installed was.. then uninstall that app and reboot.. chances are there's a conglict with that app and the apple sounds.. PZ