Do you have kmix installed and notification sounds un-muted.?

Demm.
You've stated that kmix is not necessary in the other post by Constantius24.
I'm sorry but I can see no way to activate notification sounds without Kmix. I've zeroed the notification sound slider in kmix, removed it and cannot play notification sounds. The only way I've found to enable the volume is install kmix.
Maybe I'm being "thick" and haven't found the correct way, but kmix has always worked.
Alsamixer does not show the controls to enable the sound.
PLEASE correct me if I'm wrong.

Sadly nezzie is right.

In system settings>sound>programs>notifications, even if I unmute and move the slider, immediately after releasing it returns to zero and mute.

pacman -Qs mix
local/lpsolve 5.5.2.5-1
    Mixed Integer Linear Programming (MILP) solver
local/mixxx 2.2.3-1
    Free, open source software for digital DJ'ing.
local/sdl_mixer 1.2.12-11
    A simple multi-channel audio mixer

No kmix installed here, all sounds fully working (checked in alsamixer if any is muted?):

This comment helped me:

Musikolo 2019-08-03 13:07:06 UTC
I just found that after clicking on the mute button of the Notification Sounds bar, although the sound level is 0%, notification sound is back again! This is very strange, but it worked!!

Unmuted, 0%, but sound notifications works. I just can't regulate their intensity. Probably incompatibility KCM with new PA.

Pavucontrol-qt is added, see if that helps with unmuting on your systems:

sudo pacman -U http://kaosx.tk/repo/build/pavucontrol-qt-0.14.1-1-x86_64.pkg.tar.zst

will move to all users once confirmed as helping.

OK Demm.
I've tried the pavucontrol file on a Toshiba laptop with a relatively clean installation.
Prior to installing pavucontrol the notifications slider in audio control was on an elastic, springing back to zero. After installation it is now usable.

On my other systems with installations perhaps 4 months ago there was no slider even visible until kmix was installed. As before mentioned on one system (the old PC) removing kmix stopped the notification sounds working.

I've now tried the newer PC and after removing kmix, notification sounds still worked, with the slider still visible and working.

So for me pavucontrol appears to do the job, but so did kmix.

  • demm replied to this.

    nezzie pavucontrol appears to do the job

    I'll try to figure what pulseaudio command line was run with pavucontrol, should be possible to work around this on pulseaudio install until upstream fixes the above linked bug.