I am using Version 3.0.16 and recently i disabled notifications since I wanted to keep them off while streaming with a lot of other people. However I am now unable to switch them back on.

Using the Default Sound Pack Female (tried male and text to speech as well)
Every box is checked
Play only important sounds while muted.
Playback mono to surround if available (tried other modes).

Pressed the update all bookmarks button.
I can also successfully test each of the sounds.

I am and was using a Realtek HD Audio Driver Version 6.00.0001.6662.