Guys, I'd say you're confusing the original poster quite a bit
- as I believe he has avast! 4.8, right?
The problem with Windows 7 is that the sounds are bound to a particular profile, or that the profiles change quite often, I'm not completely sure - but the truth is that the sounds are registered only in the profile that was active when avast! was installed, and they may disapper if that changes.
Bob and YoKenny were referring to avast! 5, currently in beta; the problem should be solved there, because avast! doesn't register the sounds as system events, but simply handles them itself (so, they should always be played, independent of the current profile).