In Elementary OS Freya (and probably similar, like Ubuntu 14.04), you can obtain infinite volume control by adding keyboard shortcuts to the following commands and assigning them to your keyboard volume buttons.
$ pactl — set-sink-volume @DEFAULT_SINK@ -10%
$ pactl set-sink-volume @DEFAULT_SINK@ +10%
Pushing the “Increase volume” button will now allow you to increase the volume as high as you want (until you blow your speakers). Obviously, that means that you need to use caution. Don’t try to see just how loud you can get it.
Elementary requires a log out and log in before keyboard shortcut settings are applied.