Turn the volume down to the minimum level

`Option` + `Arrow Down`

All hotkeys for QuickTime Player (MacOS)

QuickTime Player basic tasks New movie recording	`Option` + `Command` + `N`
QuickTime Player basic tasks New audio recording	`Ctrl` + `Option` + `Command` + `N`
QuickTime Player basic tasks New screen recording	`Ctrl` + `Command` + `N`
QuickTime Player basic tasks Open file	`Command` + `O`
QuickTime Player basic tasks Open location using a URL	`Command` + `U`
QuickTime Player basic tasks Close window	`Command` + `W`
QuickTime Player basic tasks Save/ Export	`Shift` + `Command` + `S`
QuickTime Player basic tasks Export for the web	`Shift` + `Command` + `E`
QuickTime Player basic tasks Minimize window	`Command` + `M`
QuickTime Player basic tasks Show Movie Inspector	`Command` + `I`
QuickTime Player basic tasks Show export Progress	`Option` + `Command` + `P`
Movie playback shortcuts Play or pause	`Space`
Movie playback shortcuts Play or pause all movies	`Command` + `Enter`
Movie playback shortcuts Stop playback and go back one frame	`Arrow Left`
Movie playback shortcuts Stop playback and go forward one frame	`Arrow Right`
Movie playback shortcuts Go to the beginning of a movie	`Option` + `Arrow Left`
Movie playback shortcuts Go to the end of a movie	`Option` + `Arrow Right`
Movie playback shortcuts Cycle through rewind speeds	`Command` + `Arrow Left`
Movie playback shortcuts Cycle through fast forward speeds	`Command` + `Arrow Right`
Movie playback shortcuts Turn volume up	`Arrow Up`
Movie playback shortcuts Turn volume down	`Arrow Down`
Movie playback shortcuts Turn the volume up to the maximum level	`Option` + `Arrow Up`
Movie playback shortcuts Turn the volume down to the minimum level	`Option` + `Arrow Down`
Movie playback shortcuts Show closed captioning	`Option` + `Command` + `T`
Movie playback shortcuts Show chapters	`Command` + `R`
Movie playback shortcuts Loop the movie	`Command` + `L`
Movie size shortcuts Enter Full screen mode	`Command` + `F`
Movie size shortcuts Exit Full screen mode	`Command` + `F` / `Esc`
Movie size shortcuts Display movie at actual size	`Command` + `1`
Movie size shortcuts Fit the movie to the screen	`Command` + `3`
Movie size shortcuts Fill the screen with the movie	`Command` + `4`
Movie size shortcuts Display the movie in panoramic mode	`Command` + `5`
Movie size shortcuts Increase the movie size	`Command` + `+`
Movie size shortcuts Decrease the movie size	`Command` + `-`
Movie editing shortcuts Undo	`Command` + `Z`
Movie editing shortcuts Redo	`Shift` + `Command` + `Z`
Movie editing shortcuts Cut	`Command` + `X`
Movie editing shortcuts Copy	`Command` + `C`
Movie editing shortcuts Paste	`Command` + `V`
Movie editing shortcuts Select all	`Command` + `A`
Movie editing shortcuts Select all excluding silent portions	`Shift` + `Command` + `A`
Movie editing shortcuts Rotate left	`Shift` + `Command` + `L`
Movie editing shortcuts Rotate right	`Shift` + `Command` + `R`
Movie editing shortcuts Flip horizontal	`Shift` + `Command` + `H`
Movie editing shortcuts Flip vertical	`Shift` + `Command` + `V`
Movie editing shortcuts Split clip	`Command` + `Y`
Movie editing shortcuts Trim	`Command` + `T`