I recently installed a fresh Kubuntu 18.04 on my Laptop, most things worked fine out of the box, including most of the multimedia keys for adjusting brightness etc. However, when I press the shortcut for Audio Up, Audio Down or Toggle Audio On/Off (FN+ F10–12), it always repeat presses them until I press a different button or put a different window in the foreground. This means It keeps toggling sound on/off or keeps adjusting volume up/down until it is at 0%/100% respectively. It also keeps playing the sound effect repeatedly. The buttons worked fine on Windows 10, so it's likely not a hardware issue. Keyboard is set to generic 105 key (intl.).
I have seen Volume Control keys duplicating on key press, but the proposed solution doesn't work for me. fn+f10–12 isn't recognised as an input by the system settings for some reason.