It’s fairly easy to add multimedia events to Gnome; that is, if you are using Rhythmbox or some another natively supported player. (Actually, that’s a lie. I don’t know whether it’s easy or not.) If you are like me, you find GUI music players a huge waste of time/resources and are using cmus or mpc or […], and in that case if you want multimedia keys to work in Gnome then you need gconf-editor:
# pacman -S gconf-editor
Fire it up and edit the following attributes located in apps->metacity->keybinding_commands thusly:
command_1 cmus-remote -r command_2 cmus-remote -C "player-pause" command_3 cmus-remote -n
Next edit the following event trigger attributes in apps->metacity->global_keybindings to:
run_command_1 <Shift>XF86AudioLowerVolume run_command_2 <Shift>XF86AudioLowerVolume run_command_3 <Shift>XF86AudioLowerVolume
… replacing the event codes as necessary (discover these with xev(1)). I only have three multimedia keys—volume up, down and mute… so I’m using Shift modally to get meself three more keys. Whatever. You know what to do.
And oh yes, you do have to log out and back into Gnome for changes to gconf to take effect. This gconf method of adding key events is limited to 12 run commands, so somebody really ought to build a GUI to do this. Ahem.
See also: Related GNOME Support Forums post.