I played around a bit in discord to get my mik not sound like an airplane. But now I got my notif sound REALLY quiet. The little sound you get when you delete something... even on 100% volume I can barely here it. Same with other notifications. The sound of increasing volume though works. Right now after I booted I was able to get the sound when deleting a file, but it does not work afterwards anymore. Also the sound when I have a powerplug in also worked the first time but not anymore. Here are some pics with my configuration in case is useful, but I don't think this has to do with anything: https://imgur.com/a/GQIsu
Seems that it got fixed on it's own O.o
Thanks for the update; closing.
Actually I found the issue. After the recent updates in KDE Neon, my notifications were also very quiet. It seems that the kcemshell5 audio stream is set very low. To fix it I had to go to the audio volume, applications, pin that, open notifications from the system settings, play a sound and then be quick enough to change the volume from around 20% to 100%.
Created attachment 111435 [details] kcemshell5 volume
Ah, this bug was fixed a long time ago.