media keys no longer work in Ubuntu 17.10
Solution 1:
I had a similar problem with the media keys (mute, play, pause) after installing the Unity interface on a clean 17.10 installation. The media keys worked fine in the login screen and on the Gnome interface desktop.
The post How to find *why* I can't bind some keyboard shortcut (or which app took it)? gave me an insight for a workaround. It seems to be some issue with compiz.
I installed Compiz Config Settings Manager
sudo apt install compizconfig-settings-manager
ccsm
and disabled the plugin Commands. After that the media keys seem to be working fine.