I've got myself an Asus k53br. and indeed all fn + random button functions work except my multimedia buttons on the arrow keys. is there any chance I can get them to work?
If the kernel recognises your keys you should be able to set them. I had a similar problem (with a different laptop) and was lucky and managed to set them The information you want is probably here at the Ubuntu help pages, but this is a run-through of what I did thanks in part to that help page.
Before pressing the keys open up
When you press the keys, the kernel should give an indication if they are recognised and will report something like the keys are not set and so give you a scancode:
Now check your free keycodes that you can associate it with by entering
Usually it's best to use a high number so as not to mess up any existing keycodes. To temporarily set the key use the value the kernel gave you and a high keycode value such as 131:
Now, if you are lucky a keysym will have automatically been assigned and you will now be able to use the button (for this session) and it will be available to set in your keyboard shortcuts program.
To make the setting permanent and so it sets it every time you reboot, add the entry to
Replace my examples with your own settings at all stages in the above process.
If the keysym has not been set, you need to go through the process detailed at the Ubuntu help page. It is quite a long process so I can't repeat it all here, but the section begins at