While I am editing, I often listen to music, because it's not always necessary to be concentrated on the video sound, when I am only working on effects. Sometimes I listen to the browser or to my media player.

It would be nice to auto-duck the music, when sound comes from kdenlive project/clip monitor.

Pulseaudio has a module for that: module-role-ducking https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/Modules/#index65h3

You can already set kdenlive's role to "phone" and e.g. firefox's role to "music" via the PULSE_PROP variable like: http://unix.stackexchange.com/posts/118524/revisions So the role "music" gets auto-ducked by "phone"

But the problem with that is, kdenlive's audio output is _always_ "on" after I clicked play at the project monitor player, so I never hear the music of Firefox after I started editing.

Maybe you can implement this feature with the ability to set off. Mumble has a similar feature for their audio output.