one thing: once you've changed the default sound scheme in Win7, you lose the avast 4 sounds as well. And restoring this default scheme won't bring you Avast sounds back, you'll have to add them manually (browsing your files through the avast interface, and this includes the voice sounds).
Now for some unknown reasons, I've noticed that sometimes the avast4 sounds get lost anyway in Win7, even without any modification of the default Windows sound scheme...Keep in mind that Avast 5 is just around the corner now, so don't spend too much time with minor issues on Avast 4.