Actually, I don't feel comfortable doing anything in safe mode.. if I just un-install that's not sufficient?
Using the Clean Uninstall method under Windows Safe Mode will get you better chances of resolving the issue.
Since you are not receiving database update tray notifications either, you have no way to know "exactly" when a sound should be heared.
Had you knew "exactly" when a sound should be heard, then I would had suggested to try the following (might had worked, but not for sure).
While an Avast notification is displayed, and a sound is supposed to be heard, you could:
1_ right-click on the "Volume" icon in your tray area;
2_ open Volume Mixer;
3_ look at the volume level and mute status of the "Avast" column.
If Avast is (supposed to be) performing a sound, a column should be displayed in the Windows Volume Mixer. This column is only displayed while the sound is heard, and that's why I say that you need to wait for some notification to be displayed (and its correspondent sound to, in theory, be heard).
After you confirm that the "Avast" column in the Windows Volume Mixer is correct (say, 100%) AND NOT in mute, you should at least wait for the next notification to actually be able to test if the sound is now active and is heard.
The only problem is, that you are not receiving database updates tray notifications either

.