I am on Ubuntu 18.04 Bionic Beaver with kernel 4.20 from here: https://kernel.ubuntu.com/~kernel-ppa/mainline/v4.20/
No idea how this happened. All of a sudden I noticed the notification popups were not producing any popping sound effects when I hit the media keys:
If I go in the top right menu and click on the sound control or scroll on it with the mousewheel I can hear the popping sound effects:
I can listen to audio without any problem. The only problem is volume change notification sounds. Usually people want those gone which is making my research quite difficult.
I reinstalled alsa and pulse today. For a moment I had no sound at all. Still don't know why. Even reinstalled my kernel and graphics drivers. After reinstalling alsa and pulse I managed to get sound back but those pesky notifications are still producing no sound. Tried changing the value of "Event Sounds" to true or false inside org.gnome.desktop.sound with dconf-editor. Still nothing. Alerts volume is at 100%. Nothing is muted inside alsa-mixer. Every other sound works: games, music, video, web, etc.
Small problem but really bugging me. Nice to have feedback when setting volume.