HI, I am running kde plasma 5.10.5 and KDE frameworks 5.37.0. My audio works but the volume control is not working correctly. It can only do either 0% (mute) or 100% (max). In between does not work. I tried this with the volume applet in the panel and the volume wheel on my keyboard.
Any help is appreciated.

Works perfectly fine here with the volume applet in the systray. Might be something wrong in your general setup, but without knowing what else is different, hard to tell. Did you modify anything in the system settings? Did you remove software or add anything more than whyt came with your distribution? And which distribution do you use?