Volume controls would not be on my list, I rarely adjust the volume.
I would just bind various applications to them. Well, I wouldn't really, it's simple enough as it is to open applications. I think these multimedia keys are highly overrated, I've got plenty and I don't use a single one.
At one point I wanted to bind a slider I have to the mouse scrollwheel, but it doesn't seem to work, because the slider isn't recognized properly.