For those saying call volume is quiet when you are on a call if you hit the menu button there is a noise suppression option that is checked. Unchecking this increases the volume.
I do not think the percentage set is the lowest it will go, you just set the baseline for that amount of light. I know for a fact that I've seen it dim lower in very dark rooms.
Android stores things in memory so they load faster and use less battery. If the system needs more ram it will clear stuff on its own. I wouldn't worry how much you have available.