There are some movies or music that sound too soft when played in QuickTime, and you don’t like to have to push the system audio volume to the loudest in order to get a decent volume.
Here’s what you need to do if the file you’re playing in QuickTime isn’t loud enough and the volume slider is already at the maximum position.
You’ll just need to hold down the Shift key while you press the Up arrow key. There won’t be any visual indicator that the volume has actually increased when you do this, so you’ll have to let your ears be the judge.
I do get the impression that the volume increase is only one level or “click” beyond the indicated maximum, and remains just slightly softer than what I get with the VLC media player at 400% volume.
To get the volume down to the normal max loudness, just hit the Up or Down arrow key once.
This technique works in iTunes too.