I've been using Karabiner for years to map the volume and media keys to a 1987 vintage Apple Extended Keyboard. Incidentally, remapping Caps Lock to Control on

I've been using Karabiner for years to map the volume and media keys to a 1987 vintage Apple Extended Keyboard. Incidentally, remapping Caps Lock to Control on that keyboard required a soldering iron and a pair of side cutters. But it works.