Quicktime has an interesting trick built in that allows you to increase the volume above 100% of what your computer normally allows. Before you click on the volume indicator in Quicktime, hold down the Shift key and continue to hold it down while you use the mouse to click on the volume control icon. You'll see that when you hold down the Shift key first, Quicktime gives you an expanded volume adjustment slider that allows you to increase the volume well above the level normally allowed by your computer.