Constant Sound Level

We are running Mopidy as ‘Jukebox’ (thx to @lesterpig for his great extension ‘Mopidy-Party’)
and are very happy with this music-server. After testing the installation for a couple of days a major
issue appears:
Due to our various input sources (youtube, soundcloud, local media, etc.) we also have different sound
levels incomming. As a consequence of this behavior we have to head to our amplifier in order to adjust
the volume manually (sometimes it must be done for each track which is not very comfortable).

Question: Is there a way to define a ‘default constant volume output level’ which is adjusted automatically
independent from the volume the incomming track has?
Can this be done with the os installed? (in our case jessie on a rpi).
Can this be done via Mopidy? Or do we need aditional extern hardware (eg. microphone) to create a
control loop?

As we are no programmers/coders, it would be great if somebody here has a hint for us to point
us to the right direction in order to find a solution for that problem.