When I press the up-volume, down-volume and mute buttons on my keyboard, nothing happens. The shortcuts option under keyboard in preferences recognizes the buttons, and org.gnome.plugins.media-keys has the right parameters, that is, XF86AudioRaiseVolume, etc. and they have been restored to defaults. I'm using
unity/xenial,now 7.4.0+16.04.20160415-0ubuntu1 amd64
on a Dell Inspiron N4110 laptop and I'm using Cairo-Dock. I have tried the solutions in Volume buttons not working in 14.04 to no avail.