I found a bug report on launchpad regarding this issue and it seems when reassigning the media keys some prefix is missing. The following lines are somewhat of a summary of what is discussed in bug report
You can issue the following command to see your current keybinding:
gsettings get org.gnome.settings-daemon.plugins.media-keys volume-up
Output should be XF86AudioRaiseVolume but when I use system setting's GUI and check I find it's value to be AudioRaiseVolume.
You can either reset them via either:
gsettings reset org.gnome.settings-daemon.plugins.media-keys volume-up
or
gsettings set org.gnome.settings-daemon.plugins.media-keys volume-up "XF86AudioRaiseVolume"
Of course the latter is more specific and might not work with all systems out there but I I was happy to find out it's just the string XF86 missing so e.g. I could bind terminal to calculator media key:
gsettings set org.gnome.settings-daemon.plugins.media-keys terminal "XF86Calculator"
For a list of possible keys to map you can use
gsettings list-keys org.gnome.settings-daemon.plugins.media-keys
play is actually play/pause !
If your setup is different you might want to figure out your media key's name and set it via dconf. I haven't done it myself but it is mentioned in a comment which in turns links back to another askubuntu-question: Where are GNOME keyboard shortcuts stored?.
Edit:
Couldn't help but to play around with it a little bit and as my keyboard is lacking some Previous/Next buttons I found CTRL+Volume-Down/CTRL+Volume-Up useful:
gsettings set org.gnome.settings-daemon.plugins.media-keys previous "<Primary>XF86AudioLowerVolume"
gsettings set org.gnome.settings-daemon.plugins.media-keys next "<Primary>XF86AudioRaiseVolume"
This worked for me on Mint18 with Cinnamon..
In synaptic package manager search for "pulse" then
mark:
pulseaudio-module-x11 for installlib cvc0 for reinstallgir1.2-cvc-1.0 for reinstall
hit apply
in terminal: sudo alsa force-reload
restart your machine.
The prob for me was that the keys were correctly bound and the OS was picking up the presses, but something was stopping it from being used for sound by cinnamon, just needed to reinstall normal settings
Best Answer
The problem is that an incorrect string is set in
dconfkey:Original (working) value:
After running "unity-control-center keyboard", and setting the shortcut for volume-up key:
The new value misses the "XF86" part of the string, and does not work.
You can get the key working again by resetting the value, with:
The bug already reported: https://bugs.launchpad.net/ubuntu/+source/unity-control-center/+bug/1302885