They worked fine in Ubuntu 17.04 and 17.10, but nothing happens in Ubuntu 18.04.
xev I can see the key presses are being interpreted correctly. I see
XF86MonBrightnessUp key press and key release events. But the volume doesn't go up or down. The screen doesn't change brightness.
Possibly related, the button which switches on and off the trackpad does work, but the on-screen indicator which used to accompany it has stopped showing. (Which looked similar to the on-screen indicators for volume and brightness under previous versions of Ubuntu.)
The volume up, down, and mute keybindings in
gnome-control-center are correct.
Lenovo ideapad 510S.
Update: I was using a Cairo Dock session. Things work fine in a GNOME on Xorg session.