MATE Mapping media buttons in Mate

I have FreeBSD 11 on a Dell Latitude E6530 and I'm using Mate for my DE. I trying to figure out how to map the media (volume, play, pause, next, back) buttons. I'm not sure where to start looking to get this done. Any help would be appreciated.
The general approach is to run xev(1) to find the keycodes, then add mappings to ~/.Xmodmap e.g.
keycode 162 = XF86AudioPlay
keycode 164 = XF86AudioStop
keycode 144 = XF86AudioPrev
keycode 153 = XF86AudioNext
and finally add the following line to ~/.xinitrc
xmodmap ~/.Xmodmap
(Restart graphics to make the changes effective).
It's not related to Mate at all, but works in any Xorg session. Mate may have its own mechanisms which I don't aware of.