well, it takes a second for me to finds out how to change the sound scheme on Ubuntu-8.10. And it was as easy as snapping your finger :D.
There you go, the snapshot of the sound preferences on my Meuligoe, that confusing me at the first time. To change or add a sound scheme, all you need to do is click on the name of the activity you'd like to change. make another click on the name of the sound, like this one: