Hm, that is odd indeed.
Actually... I think I lost those sounds too, but since I never really cared for them, I just didn't notice that they stopped playing after a while. O.o
You could try going to CTRL+K (Keyword) > type in 'Expressions' > go to 'Environment' tab > click on 'Reset all sounds to AOL Default'. (Alternatively, you could also change each of the sounds to whatever you want by using the dropdown menus on that tab.)
Hope that works.