As everyone knows, PulseAudio is pretty common in Linux for controlling audio levels. It was written by the same guy who wrote systemd. You can make up whatever politics you want around that.
PulseAudio has a configuration file called daemon.conf that is located either in ~/.config/pulse or in /etc/pulse . An important setting is flat-volumes = no
What's important to note is that Debian is, apparently, the only major distro where this setting is not set to no, which means it defaults to yes. The problem with this being at yes, is that it will cause problems with applications because changing the volume on an application like Kodi (which has its own volume control setting) will also change the system volume level and this can create issues when other applications play sound. As it happens, Kodi sets the volume level up when a new track starts playing and this causes problems when you want the volume to stay at a lower setting.
In the case of mediapc, with my reinstallation of Debian, this file already existed in my home drive with the right setting and there was no such issue. However it is useful to note that if Kodi is the active window, it will respond to keyboard volume control buttons the same way as if the Pulse Audio plugin is being used (when Kodi is not active). The difference is you are setting Kodi's internal volume control this way.
There is an active bug report open at the Debian Project (541538) which has been going for 9 years so far but may actually be getting some traction by now as all that is needed is a change in a script or something to make this setting be done properly in the default install.
