The “Option” key in QuickTime!
The left and right arrows in QuickTime let you move from frame to frame. If you add the “Option” key however, it will go back to the beginning of the video. The up and down arrow adjust the sound, but with the Option key, they’ll mute or turn the sound all the way up. The up and down arrow adjust the sound within QuickTime, not the system sound.”
(Via A New Mac Tip Every Day)