The most likely cause of this is that gnome-settings-daemon no longer supports media keys.
You can restore functionality with gnome-fallback-media-keys-helper, but it's only packaged for Zorin OS:
https://launchpad.net/~zorin-os/+archive/ubuntu/packages/+sourcepub/3778150/+listing-archive-extra
The most likely cause of this is that gnome-settings- daemon no longer supports media keys.
You can restore functionality with gnome-fallback- media-keys- helper, but it's only packaged for Zorin OS:
https:/ /launchpad. net/~zorin- os/+archive/ ubuntu/ packages/ +sourcepub/ 3778150/ +listing- archive- extra