I'm on Avast 7 Free (on Vista 32-bit) but have been irked by this, version after version, for years.
Whatever settings I have found that look like they might be relevant make no difference... when a prescheduled scan runs, at the finish a popup comes out of the system tray and (pretty much always) displays "No Threat Found." The accompanying voice only announces that the scan has been completed, but it happens AFTER the popup is halfway down. Not useful.
The audio is what catches my attention; I'm not watching my computer every minute. The voice gets me to look, only to see no information. To see what the scan result was, I have to open the UI and click a couple of times. Annoying.
What I want is for the sound to precede the visual popup by several seconds, and/or to be able to choose how long that popup stays up.
Yes, as I wrote, there are some settings that would seem to govern the popup's duration, but they don't - not at all - not this type of popup anyway.
Mostly I am posting this so the developers can see it, but if somebody knows something I don't, I'll be reading.