You can use xev and xmodmap to set your multimedia buttons.
type xev in the terminal and it should show you keycodes that map to the keys you press. So once xev started press the volume up button and note your keycode for volume up..
Then use xmodmap to associate the keycode with a function.
"xmodmap -pke" will show you your current key mappings.
"xmodmap -e keycode xxx=function" where "xxx" is the keycode number and "function" is the action you want to perform. Find a list of the functions here including the functions for volume control.
Using the "xmodmap -e keycode xxx=function" command will set the keycode for your current session, but when you reboot your laptop this keycode mapping will be gone, so create a file in your home directory .Xmodmap which will look like this but using your keycode numbers.
keycode 176 = XF86AudioRaiseVolume
keycode 174 = XF86AudioLowerVolume
keycode 160 = XF86AudioMute