How to disable the popping sound when changing the volume in KDE/Kubuntu?

Press Alt+F2 keys. A small text box will appear at top margin in center of screen with few icons. Type kmix. And now there will be a option of KMix with its logo. Click on it. And look in notification area there should be broadcasting speaker; click on it.You get dialog box with few buttons Select the button with Spanner on it. A window will open named "Configure KMix". In that window on left hand there will be General option. In General option you will uncheck the "Volume Feedback". close window. Right click on speaker in notification area select Quit. And restart KMix as described above. This should solve your problem.

The trouble with GUI-based answer is that they can't be trusted for too long...

Right-click on the tray icon:

Click on "Audio Volume Settings... Alt+D, S".

Disable "Volume feedback" (i.e., the button should be gray, not blue).

Version info:

~$ plasmashell --version
plasmashell 5.8.6
~$ kf5-config --version
Qt: 5.7.1
KDE Frameworks: 5.28.0
kf5-config: 1.0

As the new versions of KDE4 (not to mention Plasma5) have some changes in the GUI of settings, another way is the one posted in this answer.

The first solution there for KDE4 (System Settings > Common Appearance and Behaviour > Application and System Notifications > Manage Notifications > Player Settings > No audio output) didn't completely worked for me.

That may or may not be related to this bug.

Anyway, specifically for disabling the sound heard when changing the volume: click the volume tray icon and disable Event Sounds: