"Hey dude, how do I know the keycodes of the 'forward' key?" You're not a clairvoyant, and so there is a tool called xev - the X event tester. Open a console an type in

xev

Now you should see funny output in your console, especially if you move your mouse over the xev window. But now press one of your keys, e.g. the "a". The output should be something like that:

KeyPress event, serial 26, synthetic NO, window 0x3400001,
    root 0xae, subw 0x0, time 5800192, (600,303), root:(604,326),
    state 0x12, keycode 38 (keysym 0x41, A), same_screen YES,
    XLookupString gives 1 bytes:  "A"

KeyRelease event, serial 26, synthetic NO, window 0x3400001,
    root 0xae, subw 0x0, time 5800248, (600,303), root:(604,326),
    state 0x12, keycode 38 (keysym 0x41, A), same_screen YES,
    XLookupString gives 1 bytes:  "A"

You see two events here, the press and the release event. We just need one thing and that is the text

keycode 38

So you know that the "a"-Key on your keyboard has keycode 38. Now note the keycodes of your multimedia keys. If {{xev}} does not display an event if you press an extra key, than - bad luck. This HowTo won't help you.