As I mentioned before, I made a fresh install of Manjaro, and I just realised that the problem with the sound keys (volume and mute) was only partially solved. It works well until I play something with VLC. After that, those keys become irresponsive in the desktop environment. They only work if VLC is selected and playing, but there is no on-screen feedback. The “mute microfone” key stops working, as well.
Not sure if this is related to the issue, but when I play some music with VLC, two icons for Media Player appear in the top panel (I have a 13.5 KiB screenshot of that but, apparently, I am not allowed to embbed media in the post). They are somehow different, because in one the “Configure Media Player…” option brings a window with two panes, Panel View and Keyboard Shortcuts, while the other only has the Keyboard Shortcuts one.
Any idea on how to solve these issues?